emulator broken: This adb server's $ADB_VENDOR_KEYS is not set

The working fix for this particular issue is to enable Developer options on your emulator and enable "USB Debugging".

OK so eventually downloading a nexus 4 x86 image android 27, with no play store worked. Sorry if this is repetition. Long live working emulators

Combining answers from here and there gives an answer that also worked for me in IntelliJ IDEA. I assume the issue is the same, since the emulator and AVD is the same.

Option 1: Restart the emulator from AVD

Option 2: Wipe the emulator data

Option 3: Stop the emulator. Then make sure it is up to date and try again.

If it still does not work, restart the adb server and retry.

 adb kill-server && adb start-server

If it still does not work, perhaps your adb is out of date... somehow. This was the case for me. Running the following fixed that for me.

adb devices

adb server is out of date.  killing...
* daemon started successfully *
List of devices attached
emulator-5554   device

If it is now working for some emulators but not newly created ones, take note of the message in the emulator selection box: "Press Ok in the Allow USB Debugging Dialog"

Sadly, there is no such dialog. So go and enable Developer Mode in the settings of your emulated OS by tapping the Build Number several times. Then go to the Developer Settings and enable USB debugging.