Use the uploadCrashlyticsMappingFileStagingRelease task without GoogleServices

[rivella50]

When trying to build a release of my app i get the following error:

A problem was found with the configuration of task ':app:uploadCrashlyticsMappingFileStagingRelease' (type 'UploadMappingFileTask').
In plugin 'com.google.firebase.crashlytics' type 'com.google.firebase.crashlytics.buildtools.gradle.tasks.UploadMappingFileTask' property 'googleServicesResourceRoot' doesn't have a configured value.

    Reason: This property isn't marked as optional and no value has been configured.

    Possible solutions:
      1. Assign a value to 'googleServicesResourceRoot'.
      2. Mark property 'googleServicesResourceRoot' as optional.

I don't use GoogleServices plugin since i configured my app regarding Firebase by using the dart-only way. Therefore i don't have any google-services.json files lying around.
Could someone please tell me how get rid of this error in order to successfully build a release?
I use

apply plugin: "com.google.firebase.crashlytics"

in my app/build.gradle and

dependencies {
        classpath 'com.android.tools.build:gradle:4.1.3'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        classpath 'com.google.firebase:firebase-crashlytics-gradle:2.9.0'
    }

in my android/build.gradle

Thanks a lot.