diff --git a/src/tree_gui.cpp b/src/tree_gui.cpp
--- a/src/tree_gui.cpp
+++ b/src/tree_gui.cpp
@@ -231,7 +231,7 @@ static const WindowDesc _build_trees_des
 	WDP_AUTO, WDP_AUTO, 143, 184, 143, 184,
 	WC_BUILD_TREES, WC_NONE,
 	WDF_STD_TOOLTIPS | WDF_STD_BTN | WDF_DEF_WIDGET | WDF_CONSTRUCTION,
-	NULL, _nested_build_trees_widgets, lengthof(_nested_build_trees_widgets)
+	_nested_build_trees_widgets, lengthof(_nested_build_trees_widgets)
 );
 
 void ShowBuildTreesToolbar()