This appears to be a regression in the new dotnet package remove that doesn't repro in the original dotnet remove package.

Not working:
C:\test\repro>dotnet package remove System.Text.Json
Unhandled exception: Value cannot be null. (Parameter '_fileOrDirectory')

Working:
C:\test\repro>dotnet remove package System.Text.Json
info : Removing PackageReference for package 'System.Text.Json' from project 'C:\test\repro\repro.csproj'.

Working
C:\test\repro>dotnet package remove System.Text.Json --project repro.csproj
info : Removing PackageReference for package 'System.Text.Json' from project 'repro.csproj'.