The sdk platform-tools version (26.0.2) is too old to check APIs compiled with API 27; please update

I'm using IntelliJ Idea Community Edition 2017.2.5 and am building an Android app.

My problem is that even if in my app (Gradle) I have:

apply plugin: 'com.android.application'

android {
    compileSdkVersion 27
    buildToolsVersion "27.0.0"
    defaultConfig {
        applicationId "com.app.mtvtr"
        minSdkVersion 21
        targetSdkVersion 27
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}
allprojects {
    repositories {
        jcenter()
        maven {
            url "https://maven.google.com"
        }
    }
}
dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    compile 'com.android.support:appcompat-v7:27.0.0'
    compile 'com.android.support:design:27.0.0'
    compile 'com.android.support.constraint:constraint-layout:1.0.0-beta4'
    compile 'com.android.support:support-v4:27.0.0'
    testCompile 'junit:junit:4.12'
}

Also, on first build of the project, compile for the appcompat, design and support was set to 27+ (which IMHO is bad practice and is already pointed by the Studio as a bad idea), so using which tools that I have, I have set it to 27.0.1, and later to 27.0.0 just for the testing if it might work..

In my Fragments and Activities I have:

And the warning says:

The sdk platform-tools version (26.0.2) is too old to check APIs compiled with API 27; please update

Now, I know that there are topics on SO regarding old and new versions, but I have (to my belief) set the Gradle properly and I am nowhere using platform-tools version 26 or even 26.0.2...

I have tried Restarting and invalidating caches three times.

The project properly builds onto the device and everything works. But, of course, I want to fix the warning. (And not hide it like some would suggest..)

Two things that come to mind are:

1. I missed somewhere that I should have updated the version, maybe?
2. I need to start a new project from scratch.

Any ideas? :)