flutter_launcher_icons not working Unhandled exception: FormatException: Invalid number (at character 1)

I just had the same problem and solved doing this in android/app/build.gradle.

Changed:

minSdkVersion flutter.minSdkVersion
targetSdkVersion flutter.targetSdkVersion

To:

minSdkVersion 26
targetSdkVersion 30

Source

[Edit]
After that, I could not run on emulator, so I changed back the gradle file (without running flutter_launcher_icon again). Now I have the app running and the icons are right.