QmlDesigner: Add try/catch block

There seem to be rare cases in which this fails.

Change-Id: I229a9a72394ec25dca1a9cdae0b51aa36291be94
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/qmldesigner/components/itemlibrary/itemlibrarywidget.cpp

@@ -251,7 +251,11 @@ void ItemLibraryWidget::handleAddImport(int index)
             imports.append(dependencyImport);
     }
     imports.append(import);
-    model->changeImports(imports, {});
+    try {
+        model->changeImports(imports, {});
+    } catch (const Exception &e) {
+        e.showException();
+    }
 
     switchToComponentsView();
     updateSearch();