Reasons for error:
The project was opened with Unity 5.5.5.p2 before. Today, it was opened with Unity 5.5.5.p2 by accident. After opening it, I didn’t care about it until I started importing it for some time.

Solutions:
1, close the project and then open again, at this time found that the probability of this error did not report;
2. If method 1 is invalid, then:
1. Shut down the Unity project;
2. Delete the Library folder of this project;
3. Restart the project;
This error should be caused by importing unityin. UI twice when the same project was opened with different Unity versions.
Method 1 tried to solve the problem by opening the lower version of the project with the higher version, but not at other times;