If I disable the block for RegisterForStrings, I don't get the runtime error. So that makes me think it's a string handling issue. But I don't see what the problem could be.
You need to upgrade your copy of the BLE extension to the latest version. There was a change in newer versions of Android that raised a subtle bug in the older versions, and the error message there indicates you're affected by it. You need version 20240822: BluetoothLE.aix.