Usually, there are dependent packages many times in the project. Of course, v7 packages and third-party packages are lost. Sometimes they have no impact on the project. It is not a real loss, but dependency-related information cannot be viewed;
This situation must be uncomfortable for friends who have compulsory certificates. When they see the red report, they think they have reported an error. However, after working for a long time, it still doesn't work. All kinds of clearing the cache are useless...
Method: Close the current project directly
, and then
Open an existing Android Studio project Re-open your project and the solution will be perfect!