Two things.

First when we do exceptions... we don't need to get too verbose in the message.  In this case seeing a "ArgumentNullException" and that the parameter was "e" is enough.  We don't need a lesson on Visual Basic or what a ArgumentNullException means.

Second you should remove this test and exception from here.  OnTextInput is only called from your own code in KeyDown... which clearly never passes null.  It is unnecessary and not useful for MonoGame users.