When I try to connect to Bluetooth in the application version, I receive an error that the device address is invalid, but in the online test of the application, this problem does not exist and it connects to Bluetooth easily!
It would really help if you provided a screenshot of your relevant blocks, so we can see what you are trying to do, and where the problem may be.
To get an image of your blocks, right click in the Blocks Editor and select "Download Blocks as Image". You might want to use an image editor to crop etc. if required. Then post it here in the community.
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by 
Taifun.
i got the same problem... the device was connected .. when i send text to bluetooth printer..the text was printed..but error message appears.
"The specified address is not a valid Bluetooth MAC address"
thanks for helping
Switching screens breaks BlueTooth connections.