Fixed race condition when calling grab_focus after underlying vte could be closed

[TheBigS]

I found a race condition where the command I specified for the new vte had already exited by the time grab_focus was called. This caused the terminal to start cleaning up and the self.vte was None by the time grab_focus executed.

This caused a Could not call method 'has_focus()' on NoneType error. Not a huge deal because the terminal closed itself anyway. Easy fix to avoid the error by adding the None check before the has_focus call.

[mattrose]

Thanks for this