It happened to me when I accidently press the run-in debug button in android studio and then cancelled the operation very fast. Then afterwards, running the app normally generated this error.
So, what I did is, I simply restarted my system, and then opened the android studio again, and build/run the app, the app built/ran normally.

As the accepted answer suggested to stop the gradle, the gradle seems to run in the background doing some work and holding resources.

There for either you restart your system or stop the gradle with command, both will solve the problem.