bugfix: application current buffer as editor current_buffer

[cosven]

adapt to prompt_toolkit 2.0: return application current buffer as editor current_buffer

[jonathanslenders]

Thanks! This makes sense indeed, but I wonder what functionality was crashing that required this fix.

[cosven]

@jonathanslenders

Pyvim will automatically add space for me at this case, it is very useful when I edit my code. However, I do want delete those trailing whitespace when I finish editing. I try to add a command delete-trailing-whitespace, and this problem stopped me.

I submit another PR here: add command delete-trailing-whitespace