QuickDesigner: Applying no C++ backend results in soft assert

Steps to reproduce:

• create Quick Application from template (I've unchecked 'with ui.qml file')
• switch to Designer
• inside the 'Connections' widget choose Backends, click on the '+' button
• a dialog comes up with an empty combobox (Type)
• click OK

Current result:

• on the cmdline you get

  SOFT ASSERT: "false" in file /work/build/qt-creator/src/plugins/qmldesigner/qmldesignerextension/connectioneditor/backendmodel.cpp, line 234

I'd say just turn the soft assert into a valid if clause instead OR ensure that 'OK' can only get clicked if the combobox has a selected item.