title: Why do I get this error in Release build: This type is not supported on this platform INullableConverterFactory
body: <p>Using Xamarin XS v5.5, AutoMapper v3.2.1,
  I get this runtime error in Release build (not in Debug):<br>
  <code>This type is not supported on this platform INullableConverterFactory</code></p>
  
  <pre><code>[mono-rt] [ERROR] FATAL UNHANDLED EXCEPTION: System.TypeInitializationException: 
    An exception was thrown by the type initializer for AutoMapper.Mappers.MapperRegistry ---&gt;
    System.TypeInitializationException: 
    An exception was thrown by the type initializer for AutoMapper.Mappers.EnumMapper ---&gt;
    System.PlatformNotSupportedException: This type is not supported on this platform INullableConverterFactory
  [mono-rt]   at AutoMapper.Internal.PlatformAdapter.Resolve[INullableConverterFactory] (Boolean throwIfNotFound) [0x00000] in &lt;filename unknown&gt;:0 
  [mono-rt]   at AutoMapper.Mappers.EnumMapper..cctor () [0x00000] in &lt;filename unknown&gt;:0 
  [mono-rt]   --- End of inner exception stack trace ---
  [mono-rt]   at AutoMapper.Mappers.MapperRegistry..cctor () [0x00000] in &lt;filename unknown&gt;:0 
  [mono-rt]   --- End of inner exception stack trace ---
  [mono-rt]   at AutoMapper.Mapper.Reset () [0x00000] in &lt;filename unknown&gt;:0 
  [mono-rt]   at AutoMapper.Mapper.Initialize (System.Action`1 action) [0x00000] in &lt;filename unknown&gt;:0 
  [mono-rt]   at MyApp.Android.MainActivity.ConfigMapper () [0x00000] in &lt;filename unknown&gt;:0 
  [mono-rt]   at MyApp.Android.MainActivity.OnCreate (Android.OS.Bundle bundle) [0x00000] in &lt;filename unknown&gt;:0 
  [mono-rt]   at Android.App.Activity.n_OnCreate_Landroid_os_Bundle_ (IntPtr jnienv, IntPtr native__this, IntPtr native_savedInstanceState) [0x00000] in &lt;filename unknown&gt;:0 
  [mono-rt]   at (wrapper dynamic-method) object:3cc570cf-1f33-4513-b6e4-0e910820d0ba (intptr,intptr,intptr)
  </code></pre>
  
  <p>What could the issue be?</p>