fix autobug while shutting down interface

app/src/main/java/com/geeksville/mesh/service/BluetoothInterface.kt

@@ -313,7 +313,7 @@ class BluetoothInterface(val service: RadioInterfaceService, val address: String
     var fromNumChanged = false
 
     private fun startWatchingFromNum() {
-        safe!!.setNotify(fromNum, true) {
+        safe?.setNotify(fromNum, true) {
             // We might get multiple notifies before we get around to reading from the radio - so just set one flag
             fromNumChanged = true
             debug("fromNum changed")