Check for null pointer in get_column_width(0)

(cherry picked from commit fa6fd3a2d075c9e9b831f22cf8a5b5de8d24f168)

scene/gui/tree.cpp

@@ -3370,9 +3370,13 @@ int Tree::get_column_width(int p_column) const {
 	if (!columns[p_column].expand)
 		return columns[p_column].min_width;
 
+	int expand_area = get_size().width;
+
 	Ref<StyleBox> bg = cache.bg;
 
-	int expand_area = get_size().width - (bg->get_margin(MARGIN_LEFT) + bg->get_margin(MARGIN_RIGHT));
+	if (bg.is_valid()) {
+		expand_area -= bg->get_margin(MARGIN_LEFT) + bg->get_margin(MARGIN_RIGHT);
+	}
 
 	if (v_scroll->is_visible_in_tree())
 		expand_area -= v_scroll->get_combined_minimum_size().width;