Fix #4588 (crash Control::_set_anchor)

Thanks to @vnen for testing this

scene/gui/control.cpp

@@ -1287,14 +1287,13 @@ void Control::set_anchor(Margin p_margin,AnchorType p_anchor, bool p_keep_margin
 
 void Control::_set_anchor(Margin p_margin,AnchorType p_anchor) {
 	#ifdef TOOLS_ENABLED
-	SceneTree *st=OS::get_singleton()->get_main_loop()->cast_to<SceneTree>();
-	if (st && st->is_editor_hint()) {
+	if (is_inside_tree() && get_tree()->is_editor_hint()) {
 		set_anchor(p_margin, p_anchor, EDITOR_DEF("2d_editor/keep_margins_when_changing_anchors", false));
 	} else {
-		set_anchor(p_margin, p_anchor);
+		set_anchor(p_margin, p_anchor, false);
 	}
 	#else
-	set_anchor(p_margin, p_anchor);
+	set_anchor(p_margin, p_anchor, false);
 	#endif
 }
 

tools/editor/editor_settings.cpp

@@ -459,6 +459,8 @@ void EditorSettings::_load_defaults(Ref<ConfigFile> p_extra_config) {
 	set("2d_editor/bone_color2",Color(0.75,0.75,0.75,0.9));
 	set("2d_editor/bone_selected_color",Color(0.9,0.45,0.45,0.9));
 	set("2d_editor/bone_ik_color",Color(0.9,0.9,0.45,0.9));
+	
+	set("2d_editor/keep_margins_when_changing_anchors", false);
 
 	set("game_window_placement/rect",0);
 	hints["game_window_placement/rect"]=PropertyInfo(Variant::INT,"game_window_placement/rect",PROPERTY_HINT_ENUM,"Default,Centered,Custom Position,Force Maximized,Force Full Screen");