Fix merge persons when removed person was the Home (default) person (#…

…619)

Fixes #10597

If a person merge removes the default (Home) person, the previous code started having HandleError crashes.
This checks for and corrects this issue.
Did not see the issue with 4.2.x because there were no HandleErrors.

gramps/gen/merge/mergepersonquery.py

@@ -182,5 +182,7 @@ def __execute(self, family_merger, trans):
                 self.database.commit_family(family, trans)
             parent_list.append(parents)
 
+        if self.database.get_default_handle() == old_handle:
+            self.database.set_default_person_handle(None)
         self.database.remove_person(old_handle, trans)
         return family_merge_ok