fix: add guards to check for deleted parent windows

[Anna-log7]

Fixes #284 and #253

The line moved on line 356 is to get rid of the error for Vim:E444: Cannot close last window, which will show up if the parent is invalid.

The added check on line 307 is to fix the error: E565: Not allowed to change text or change window which errors on line 309 when the parent is invalid.

Both cases can be reproduced by having 2 or more buffers open, opening Trouble, then using :bdelete to delete your current "parent" buffer