For me it was crashing on opening any storyboard or xib in my app. I have created a new single view app project, opened its storyboard without a problem, reopened my own project - and it started opening storyboards and xibs as usual. Probably a caching issue of some sort..

Here are the details of a crash I was experiencing:

Crashed Thread:        0  Dispatch queue: com.apple.main-thread

Exception Type:        EXC_CRASH (SIGABRT)
Exception Codes:       0x0000000000000000, 0x0000000000000000
Exception Note:        EXC_CORPSE_NOTIFY

Application Specific Information:
Sending _openAs: to <IDEDocumentController: 0x7f944580a970> from <NSMenuItem: 0x7f94249528d0 Interface Builder - Storyboard>
ProductBuildVersion: 9A235
ASSERTION FAILURE in /Library/Caches/com.apple.xbs/Sources/IDEInterfaceBuilder/IDEInterfaceBuilder-13196/InterfaceBuilderKit/IssueProvider/IBIssueProvider.m:72
Details:  castedInstance should be an instance inheriting from IBDocument, but it is IDEPegasusSourceEditor.SourceCodeDocument
Object:   <IBIssueProvider: 0x7f94366a73d0>
Method:   -updatedIssuesForDocument:

Another crash that I have experienced, and that got fixed the same way:

Crashed Thread:        0  Dispatch queue: com.apple.main-thread

Exception Type:        EXC_CRASH (SIGABRT)
Exception Codes:       0x0000000000000000, 0x0000000000000000
Exception Note:        EXC_CORPSE_NOTIFY

Application Specific Information:
Sending openClickedNavigableItemAction: to <IDEStructureNavigator: 0x7ff718acde20 representing: <DVTExtension 0x7ff70acf22e0: Structure Navigator (Xcode.IDEKit.Navigator.Structure) v0.1 from com.apple.dt.IDEKit>> from <IDENavigatorOutlineView: 0x7ff7169ba340>
ProductBuildVersion: 9A235
ASSERTION FAILURE in /Library/Caches/com.apple.xbs/Sources/IDEFrameworks/IDEFrameworks-13247/IDEKit/Editor/IDEEditorContext.m:1935
Details:  Unexpected document in IDEDocumentController:<IBXIBDocument: 0x7ff6fb013a00>(  0   -[IDEEditorDocument init] (in IDEKit)

I had the same issue, but in my case it was not solved by removing nil keys. As highlighted by Vlad Valentin, it was related to a UITabBarViewController I had in my Storyboard.

The issue, as stated in the error thrown in the Xcode crash report:

UNCAUGHT EXCEPTION (NSInternalInconsistencyException): Could not find class named UIImage

related to a background image I had assigned to the tabBar. Once I removed the image from the Storyboard source code, I could then open Interface Builder again.

Remove:

<tabBar key="tabBar" contentMode="scaleToFill" backgroundImage="Tabbar background" id="W28-zg-YXA">

When I tried re-adding the image using Interface Builder, Xcode threw the same exception. I have resorted to loading the image programmatically. Hope this helps, and will save some people having to fully re-create their UITabBarViewController's.

I'm getting a similar issue. In my case, I've found that it consistently crashes when opening via the file explorer, whereas opening via Open Quickly (CMD+SHIFT+O) does not cause XCode to crash.

The issue is a nil key tag (at least for my case)

<nil key="simulatedBottomBarMetrics"/>

or more generally

<nil key="<generic_name>"/>

That has its counterpart

<simulatedToolbarMetrics key="simulatedBottomBarMetrics"/>

or more generally

<<generic_name> key="generic_name"/>

Removing all of them solves the issue.

1. Open the storyboard as Source Code
2. Search and remove: <nil key="simulatedBottomBarMetrics"/> or any <nil key="<generic_name>"/>(replace <generic_name> with your particular tag name)
3. Search and remove their counter part: <simulatedToolbarMetrics key="simulatedBottomBarMetrics"/> or any << generic_name > key="generic_name"/> (replace <generic_name> with your particular tag name)
4. Save and open your storyboard normally
5. Enjoy

EDIT:

This tags seems to be related and are in the Storyboard Source Code of UITabBarControllers however their counterparts are scattered around UIViewControllers connected to the UITabBarController. Can be one, two or more levels below.

I found a solution for my case. It was crashing from the UITabBarViewController.

1. Open the storyboard as source code
2. Search for your UITabBarViewController
3. Remove the tab bar
4. Open storybard as Interface builder
5. Create new tab bar and connect the root view controllers you need

As Michele Schillaci said, you can try to remove some view controller and check if that solved the problem.

There is a known bug with tab bars containing system tab bar items. The Xcode 9.1 release notes acknowledge this, and suggest a workaround:

Interface Builder

A UITabBarItem configured as a system item in a storyboard or xib causes a build failure. (34197132)

Workaround: Configure as a custom item in Interface Builder, then reset the tab bar item to the desired system item in the awakeFromNib method of the view controller.

I removed <nil key ="...."> but it didn't work. So i tried that (From @Vlad Valentin):

1. Open the storyboard as source code.

2. Search for tabBarController

    <scene sceneID="RIF-kY-Kwk">
       <objects>
           <tabBarController storyboardIdentifier="mainViewController" id="p5V-Jj-erc" customClass="MainViewController" customModule="yourModule" customModuleProvider="target" sceneMemberID="viewController">
               <tabBar key="tabBar" contentMode="scaleToFill" insetsLayoutMarginsFromSafeArea="NO" id="kWD-Cd-EAU">
                   <rect key="frame" x="0.0" y="0.0" width="375" height="49"/>
                   <autoresizingMask key="autoresizingMask"/>
                   <color key="backgroundColor" white="0.0" alpha="0.0" colorSpace="calibratedWhite"/>
               </tabBar>
               <connections>
                   <segue destination="FQU-DF-g8S" kind="relationship" relationship="viewControllers" id="wf7-sS-3aP"/>
                   <segue destination="cVl-bj-mqD" kind="relationship" relationship="viewControllers" id="A5l-iP-1pv"/>
                   <segue destination="yE9-HB-Hqo" kind="relationship" relationship="viewControllers" id="U4m-N7-DeC"/>
                   <segue destination="PmG-7U-DMm" kind="relationship" relationship="viewControllers" id="u4J-Y7-Z51"/>
               </connections>
           </tabBarController>
           <placeholder placeholderIdentifier="IBFirstResponder" id="APY-5z-hrA" userLabel="First Responder" sceneMemberID="firstResponder"/>
       </objects>
       <point key="canvasLocation" x="-3126" y="1236"/>
   </scene>
   

3. storyboardIdentifier="mainViewController", customClass="MainViewController" and <connections>..</connections> are important for you. Copy and paste it.

4. Remove scene.
5. Now you can open storyboard with Interface Builder.
6. Add Tab Bar Controller (Drag Drop and remove child view controller.)
7. Go Stroyboard source code and paste storyboardIdentifier, customClass and connections.

Easy fix:

Just put insetsLayoutMarginsFromSafeArea="NO" on <tabBar key="tabBar" contentMode="scaleToFill" id="ytw-4N-0RV"> and remove all <nil key ="...."> from your storyboard source code.

E.g:

<tabBarController id="mhN-Gc-eXK" sceneMemberID="viewController" restorationIdentifier="tabbar" storyboardIdentifier="tabbar" customClass="TabbarMainViewController">
    <tabBar key="tabBar" contentMode="scaleToFill" insetsLayoutMarginsFromSafeArea="NO" id="ytw-4N-0RV">
        <rect key="frame" x="0.0" y="0.0" width="320" height="49"/>

I found that the previously installed versions of XCode were the root of the problem. I thought I only had one version installed but upon right-clicking an XCode project file, I saw that under 'Open With..', there were tons of versions. There were old beta versions of XCode 9, as well as various versions of the stable release and even XCode 8.

None of these versions could be found in my mac's Applications directory, so I realized that they were dormant in the Downloads directory. What I did was delete all of them and boom, storyboard not freezing XCode 9.3 anymore.

I replaced this

<tabBar key="tabBar" contentMode="scaleToFill" translucent="NO" backgroundImage="background_tabbar" selectionIndicatorImage="background_tabbar_item" id="Z9r-yE-1Zs">

on that

<tabBar key="tabBar" contentMode="scaleToFill" id="Z9r-yE-1Zs">

and Xcode doesn't crash anymore when I try to open the storyboard

So I had an issue whereby as soon as I opened a storyboard in XCode 9 the file was modified (not unusual for XCode) - but then whenever I ran the app with Exception breakpoints enabled it crashed. Hitting continue from the debugger allowed the app to then run normally.

Looking at what XCode has modified in the storyboard file, it turned that at the top of the file, before the 'scenes' xml element, it was adding:

<customFonts key="customFonts">
    <array key="GillSans.ttc">
        <string>GillSans</string>
        <string>GillSans-Light</string>
    </array>
</customFonts>

Removing this stopped the weird crash issue, but modifying the storyboard keeps adding the code back in. It appears the solution would be to remove all custom fonts from the storyboard and set them in code - or hope this is just a weird bug which will be fixed in a later XCode version.

I was having the same issue: a storyboard created in Xcode 8 would hang in Xcode 9. After taking my Storyboard apart by exporting the views one by one (using Editor / Refactor to Stoyboard - a feature I didn't know existed until I was struck by this issue) I found that two specific views were causing Xcode to hang.

In both these views, I had added more than one item to a UITabBar - a button on the left, a flexible space and a button on the right. If Xcode tried to display this, it would hang. If I quickly clicked to any other Storyboard once these "poisoned" views were seen, it would also hang.

The solution is therefore to open the project in Xcode 8, remove all but one item from the TabBar, delete any Derived Data folders, quit Xcode 8, load Xcode 9, do a Product / Clean, and if you're lucky, you'll be good to go.