fix(find_in_files): crash when selecting with RMB

in replace mode, if no search result has been selected and RMB is pressed on a checkbox, the editor
crashes in editor/find_in_files.cpp:712 because item is NULL

editor/find_in_files.cpp

@@ -547,6 +547,7 @@ FindInFilesPanel::FindInFilesPanel() {
 	_results_display->connect("item_edited", this, "_on_item_edited");
 	_results_display->set_hide_root(true);
 	_results_display->set_select_mode(Tree::SELECT_ROW);
+	_results_display->set_allow_rmb_select(true);
 	_results_display->create_item(); // Root
 	vbc->add_child(_results_display);