I guess it was because after run instrumented test file so become can't open Android Studio's emulator? But I am not sure.
Error message: Launching 'app' on No Devices. Error while waiting for device: The emulator process for AVD Pixel_4_XL_API_30 was killed.
I used follow solution solved this problem.↓
Tools -> SDK Manager -> SDK Tools (tab) -> Deselect 'Android Emulator' -> OK
Now, when you try to run your app, or launch device from AVD Manager, it will give the 'Install Emulator' error -> Click Ok. This will automatically download the correct version.
I experienced the same problem with emulator version 30.6.4 and Android Studio Arctic Fox 2020.3.1 Canary 15. I tried switching from Intel HAXM to Windows Hyper-V and re-installed all android build tool and emulator packages. Afterwards it worked for a day.
The second thing I tried is manually installing an older version (30.5.5) of the emulator which you can get from here. Now Android Studio does not recognize my manual installation of the emulator and I can not start the AVD from the UI. However I can start it via cmd and install apps from within android studio without crashes (at least for the moment).
Start your AVD with the following command (android/sdk/emulator):
emulator -avd <name-with-_-instead-of-spaces>