DMError: Only one instance of an application may be running at once

[modeswitch]

Failed to launch application
Traceback (most recent call last):
File "/home/ack/work/speedtests/client/AndroidBrowserControllers.py", line 121, in launch
return self.dm.launchApplication(self.browserPackage, self.browserActivity, "android.intent.action.VIEW", extras=extras, url=url)
File "/home/ack/work/speedtests/client/mozdevice/droid.py", line 39, in launchApplication
raise DMError("Only one instance of an application may be running "
DMError: Only one instance of an application may be running at once