Qt Quick Designer does not show a QML component if you import an unknown module. This would be an understandable behavior if the module actually didn't exist, but it's not passable if the module does exist but comes from a C++ class.

Example:

• main.qml

  import QtQuick 2.0
  
  MyComponent {
    x: 0
    y: 0
    height: 200
    width: 200
  }
  

• MyComponent.qml

  import QtQuick 2.0
  import Testing 1.0
  
  Image {
     x: 0
     y: 0
     source: "myimage.png"
  }
  

• main.cpp

  class TestClass;
  ...
  
  QGuiApplication app(argc, argv);
  QtQuick2ApplicationViewer viewer;
  qmlRegisterType<TestClass>("Testing", 1, 0, "Test");
  viewer.setMainQmlFile(QStringLiteral("main.qml"));
  

  end of example

If you try to edit main.qml on Qt Quick Designer, it simply doesn't show the component MyModule that should be inside it. However, if I comment the line "import Testing 1.0" on MyComponent.qml, it shows the component once again.