Do not crash in GrepOutputModel

In case there are no matches you can still trigger the replace.
=> Crash in GrepOutputmodel because it assumes the root item of the
model is still there.

Fixes:
* Disable the replace button in case there are no matches
* Do not crash in GrepoutModel in case the root item is invalid (call
  safety)

BUG: 303329
REVIEW: 107548