Add editor warnings to ButtonSelection

user_interface/button_selection.gd

@@ -16,7 +16,14 @@ var item_count: int:
 		return _button_group.get_buttons().size()
 
 func _get_configuration_warnings() -> PackedStringArray:
-	return PackedStringArray()
+	var warnings := PackedStringArray()
+
+	for child in get_children():
+		warnings.append(
+			"ItemSelection can only have children of type Button, but {0} is of type {1}".format(
+				[child.name, child.get_class()]))
+
+	return warnings
 
 ##
 ## Adds a new item with no text and only [code]icon[/code] as the icon to the selection.