ModelEditor: Introduce QMT_ASSERT

This change shall solve a lot of Coverity findings

Change-Id: I1e699f7363426e9b6008fc77d3f498fe3d968b4f
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/libs/modelinglib/qmt/diagram_scene/items/componentitem.cpp

@@ -237,7 +237,7 @@ void ComponentItem::relationDrawn(const QString &id, const QPointF &toScenePos,
 bool ComponentItem::hasPlainShape() const
 {
     auto diagramComponent = dynamic_cast<DComponent *>(object());
-    QMT_CHECK(diagramComponent);
+    QMT_ASSERT(diagramComponent, return false);
     return diagramComponent->isPlainShape();
 }