I was using it normally, but suddenly there was a problem.
Please search the community, this bug has been raised several times
This is my work-around. Re-name the existing BluetoothClient1 Block, e.g 'BluetoothClientOld'. Drag a new one in. Go through your Blocks and simply swap the old for the new. Delete the old BluetoothClient Block. Done.
This problem should be resolved with App Inventor release nb190a.