Have the same issue after adding Xamarin.GooglePlayServices.Ads package.
14>C:\Program Files\dotnet\packs\Microsoft.Android.Sdk.Windows\35.0.101\tools\Xamarin.Android.Common.targets(1435,3): error XALNS7001: System.NullReferenceException: Object reference not set to an instance of an object.
14>C:\Program Files\dotnet\packs\Microsoft.Android.Sdk.Windows\35.0.101\tools\Xamarin.Android.Common.targets(1435,3): error XALNS7001: at MonoDroid.Tuner.AddKeepAlivesStep.AddKeepAlives(TypeDefinition type) in /Users/runner/work/1/s/xamarin-android/src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/AddKeepAlivesStep.cs:line 109
14>C:\Program Files\dotnet\packs\Microsoft.Android.Sdk.Windows\35.0.101\tools\Xamarin.Android.Common.targets(1435,3): error XALNS7001: at MonoDroid.Tuner.AddKeepAlivesStep.ProcessType(TypeDefinition type) in /Users/runner/work/1/s/xamarin-android/src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/AddKeepAlivesStep.cs:line 65
14>C:\Program Files\dotnet\packs\Microsoft.Android.Sdk.Windows\35.0.101\tools\Xamarin.Android.Common.targets(1435,3): error XALNS7001: at MonoDroid.Tuner.AddKeepAlivesStep.AddKeepAlives(AssemblyDefinition assembly) in /Users/runner/work/1/s/xamarin-android/src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/AddKeepAlivesStep.cs:line 56
14>C:\Program Files\dotnet\packs\Microsoft.Android.Sdk.Windows\35.0.101\tools\Xamarin.Android.Common.targets(1435,3): error XALNS7001: at Xamarin.Android.Tasks.LinkAssembliesNoShrink.DoRunTask(ITaskItem source, ITaskItem destination, RunState runState, WriterParameters writerParameters) in /Users/runner/work/1/s/xamarin-android/src/Xamarin.Android.Build.Tasks/Tasks/LinkAssembliesNoShrink.cs:line 146
14>C:\Program Files\dotnet\packs\Microsoft.Android.Sdk.Windows\35.0.101\tools\Xamarin.Android.Common.targets(1435,3): error XALNS7001: at Xamarin.Android.Tasks.LinkAssembliesNoShrink.RunTask() in /Users/runner/work/1/s/xamarin-android/src/Xamarin.Android.Build.Tasks/Tasks/LinkAssembliesNoShrink.cs:line 108
14>C:\Program Files\dotnet\packs\Microsoft.Android.Sdk.Windows\35.0.101\tools\Xamarin.Android.Common.targets(1435,3): error XALNS7001: at Microsoft.Android.Build.Tasks.AndroidTask.Execute() in /Users/runner/work/1/s/xamarin-android/external/xamarin-android-tools/src/Microsoft.Android.Build.BaseTasks/AndroidTask.cs:line 25
14>Done building project "Application.csproj" -- FAILED.
I've solved that by adding this line to the .csproj:
<PropertyGroup>
<!-- everythin else above -->
<AndroidAddKeepAlives>False</AndroidAddKeepAlives>
</PropertyGroup>