SLA gizmo: fixed a crash with multiple selection

src/slic3r/GUI/Gizmos/GLGizmoSlaSupports.cpp

@@ -59,7 +59,7 @@ bool GLGizmoSlaSupports::on_init()
 
 void GLGizmoSlaSupports::set_sla_support_data(ModelObject* model_object, const Selection& selection)
 {
-    if (selection.is_empty()) {
+    if (! model_object || selection.is_empty()) {
         m_model_object = nullptr;
         return;
     }