Remove dummy pad from group before deleting

The dummy pad doesn't need group information, and it asserts when it has
it and is deleted.

Fixes KICAD-3B9

3d-viewer/3d_canvas/create_3Dgraphic_brd_items.cpp

@@ -364,6 +364,9 @@ void BOARD_ADAPTER::createPadWithMargin( const PAD* aPad, CONTAINER_2D_BASE* aCo
         dummy.SetSize( VECTOR2I( dummySize.x, dummySize.y ) );
         dummy.TransformShapeToPolygon( poly, aLayer, 0, maxError, ERROR_INSIDE );
         clearance = { 0, 0 };
+
+        // Remove group membership from dummy item before deleting
+        dummy.SetParentGroup( nullptr );
     }
     else
     {