[Qt] What should I do if the "Design" button in QtCreator is gray?

In QtCreator, xxx.qml and xxx.ui.qml files can be opened through "Design", but xxx.qml often does not have any 2D or 3D display effect. But if the corresponding plug-in is not installed when QtCreator is newly installed, xxx.qml and xxx.ui.qml cannot be opened.

Solution:

Open in turn: Help -> About Plugins. Find the place shown in the picture below and check it:

Then restart QtCreator.