fatal error: 'Flutter/Flutter.h' file not found when running for iOS

I have a flutter app which was using only the android side. Now I want to run it on a iPad but it throws this error:

Error output from Xcode build:
↳
    2021-04-19 11:31:26.233 xcodebuild[21793:154600]  DVTAssertions: Warning in /Library/Caches/com.apple.xbs/Sources/DVTiOSFrameworks/DVTiOSFrameworks-17705/DTDeviceKitBase/DTDKRemoteDeviceData.m:371
    Details:  (null) deviceType from 00008101-00126D441E38001E was NULL when -platform called.
    Object:   <DTDKMobileDeviceToken: 0x7f8196cb0f70>
    Method:   -platform
    Thread:   <NSThread: 0x7f8196c6dc80>{number = 8, name = (null)}
    Please file a bug at https://feedbackassistant.apple.com with this warning message and any useful information you can provide.
    2021-04-19 11:31:26.651 xcodebuild[21793:154569]  DVTAssertions: Warning in /Library/Caches/com.apple.xbs/Sources/DVTiOSFrameworks/DVTiOSFrameworks-17705/DTDeviceKitBase/DTDKRemoteDeviceData.m:371
    Details:  (null) deviceType from 00008101-00126D441E38001E was NULL when -platform called.
    Object:   <DTDKMobileDeviceToken: 0x7f8196cb0f70>
    Method:   -platform
    Thread:   <NSThread: 0x7f8196a38370>{number = 5, name = (null)}
    Please file a bug at https://feedbackassistant.apple.com with this warning message and any useful information you can provide.
    2021-04-19 11:31:26.835 xcodebuild[21793:154569]  DVTAssertions: Warning in /Library/Caches/com.apple.xbs/Sources/DVTiOSFrameworks/DVTiOSFrameworks-17705/DTDeviceKitBase/DTDKRemoteDeviceData.m:371
    Details:  (null) deviceType from 00008101-00126D441E38001E was NULL when -platform called.
    Object:   <DTDKMobileDeviceToken: 0x7f8196cb0f70>
    Method:   -platform
    Thread:   <NSThread: 0x7f8196a38370>{number = 5, name = (null)}
    Please file a bug at https://feedbackassistant.apple.com with this warning message and any useful information you can provide.
    ** BUILD FAILED **


Xcode's output:
↳
    In file included from /Users/interno/Documents/FlutterSDK/flutter/.pub-cache/hosted/pub.dartlang.org/url_launcher-5.7.10/ios/Classes/FLTURLLauncherPlugin.m:7:
    /Users/interno/Documents/FlutterSDK/flutter/.pub-cache/hosted/pub.dartlang.org/url_launcher-5.7.10/ios/Classes/FLTURLLauncherPlugin.h:5:9: fatal error: 'Flutter/Flutter.h' file not found
    #import <Flutter/Flutter.h>
            ^~~~~~~~~~~~~~~~~~~
    1 error generated.
    Command CompileSwift failed with a nonzero exit code
    Command CompileSwift failed with a nonzero exit code
    note: Using new build system
    note: Building targets in parallel
    note: Planning build
    note: Constructing build description

Could not build the precompiled application for the device.

Things I already tried:

• Run flutter clean
• pod install
• pod update
• Delete the 'iOS' folder and run 'flutter create'

What I have to do to fix it?