Open *.ui file by Qt Designer in VS, but the window turned off automatically.

When I opened the *.ui file, the window of Qt Designer showed up. However, after waiting for about 5 seconds, it turned off automatically. And an error message showed up:

Cannot open file. (qtwidgetsapplication2.ui)

When I opened the *.ui file and turned off the windows of Qt Designer immediately, another error message showed up:

Object reference not set to an instance of an object.