I also encountered this error on running msbuild, but I do not agree with the accepted solution as this was encountered even when using the Developer Command Prompt for VS 2019 and with msbuild added to the path. So I don't believe it is a path issue.
I managed to narrow down the error to the value/property which is null. This is the "SolutionPackageMapFilePath" passed to the SolutionPackagerTask within the Microsoft.PowerApps.MSBuild.Solution.Targets file:
I printed out the vars using the <Message /> tag:
As one can see the MappingFilePath and LocalTemplate are essentially null values. Based on tests, the former value cannot be null or absent for whatever reason. The error only goes away when one supplies an actual path to a Map.xml file. However, this opens a can of worms in terms of getting into how to configure and use mapping files for the build. There doesn't seem to be an option to say no to using a mapping file in this version of the pac client.
However, since the built files are still created and added to Solutions\PCF_HelloWorld\obj\Debug\Metadata (where "PCF_HelloWorld" is the name of your component) one can apply the pac solution pack command to still create the final zip file required for upload to the Solutions PowerApps portal. For example:
pac solution pack -z C:\Users\Public\Documents\User\PowerAppComponents\Solutions\PCF_HelloWorld\bin\Debug\HelloWorld.zip -f C:\Users\Public\Documents\User\PowerAppComponents\Solutions\PCF_HelloWorld\obj\Debug\Metadata
I propose this as a work-around rather than a direct solution therefore. Of course if anyone else can shed any light on this error (@DianaBirkelbach, @joem , @cchannon , @Aura, @mick4 ) I would be very interested.
Many thanks
