Unable to publish Xamarin App to iPhone. It will not compile - xamarin

Xamarin Forms v4.3.0.947036 VS 2019
I made a dummy App (Master/Detail Xamarin template) and published it to an iPhone connected to a MAC on my network. Runs fine.
I have another App (my main App) that functions fine on both Android/iOS emulators and Android Phone.
However, when I try and publish it to the iPhone (iOS 12.4) (same setup as with the Dummy App) I get the following error when I compile:
Can't resolve the reference 'System.Void Xamarin.Forms.Log::Warning(System.String,System.String)', referenced from the method 'System.Void Xamarin.Forms.Pages.BaseDataSource/<Initialize>d__22::MoveNext()' in 'Xamarin.Forms.Core, Version=2.0.0.0, Culture=neutral, PublicKeyToken=null'.
This is the full error in the output window:
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Xamarin\iOS\Xamarin.iOS.Common.targets(842,3): error MT2101: Can't resolve the reference 'System.Void Xamarin.Forms.Log::Warning(System.String,System.String)', referenced from the method 'System.Void Xamarin.Forms.Pages.BaseDataSource/<Initialize>d__22::MoveNext()' in 'Xamarin.Forms.Core, Version=2.0.0.0, Culture=neutral, PublicKeyToken=null'.
Does anyone have any ideas on what could possibly be causing the error?

The problem was with the pre-release 'themes'. I removed all references to them from any Projects and the error disappeared.
Reference: https://forums.xamarin.com/discussion/140586

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