As for the problem of “Session ‘app’ : error-Launching Activity”, there are two online solutions, but none of them can launch my own

The basic solutions are

File-> Settings-> Build, Execution,Deployment find Instant Run and remove the pre-instant Run option from tools-& gt; Android -> Sync Project with Gradle Files

Another is that the APP seems to be uninstalled, and the icon can’t be found on the desktop of the phone, but there is still caching; Open the command line directly

adb shell pm uninstall <YourAppPackageName>

Use the command to unload residual applications.

The 2019-10-28 supplement
The command line mentioned above still has some problems in the case of multiple users in the system, which may lead to unclean uninstall, too low version number or inconsistent signature, resulting in the new APK can’t be installed, and can’t be installed when using Android Studio to directly run. At this point, we can adjust the version number to a large number, and then repackage, with adb install XXX to cover the installation; Install successfully, then manually uninstall, then go to run is OK. More operational references

adb shell 
pm --help