Add assert to InstrumentItem::setBackgroundType

GUI/Models/InstrumentItems.h

@@ -174,6 +174,9 @@ public:
 template <typename T>
 T* InstrumentItem::setBackgroundType()
 {
+    static_assert(std::is_base_of<BackgroundItem,T>::value,
+                  "Class must be derived from BackgroundItem");
+
     return setGroupPropertyType<T>(P_BACKGROUND);
 }