Fix incorrect static typing in 2.5D demo

misc/2.5d/addons/node25d/node25d_plugin.gd

@@ -3,7 +3,7 @@ extends EditorPlugin
 
 const MainPanel = preload("res://addons/node25d/main_screen/main_screen_25d.tscn")
 
-var main_panel_instance: Panel
+var main_panel_instance: VBoxContainer
 
 
 func _enter_tree() -> void: