If you have a SAMSUNG device, don’t bother – SAMSUNG broke run-as by dropping the setuid flag (so run-as has no chance of switching to a different identity).
Also don’t bother trying the Smart Switch “reinitialize device” workaround, it won’t work until SAMSUNG fixes it in the firmware (so it is worth updating to the latest version).
Use some other method to access your app’s data, like adb backup:
adb backup -f data.ab <my.package>
dd if=data.ab bs=24 skip=1 | openssl zlib -d > data.tar
Instead of dd you can use the abe tool, which can also write .ab files.