fix: removeObservers when onStop is called (UI not visible)

app/src/main/java/com/geeksville/mesh/MainActivity.kt

@@ -210,10 +210,6 @@ class MainActivity : AppCompatActivity(), Logging {
             tab.icon = ContextCompat.getDrawable(this, tabInfos[position].icon)
         }.attach()
 
-        model.connectionState.observe(this) { connected ->
-            updateConnectionStatusImage(connected)
-        }
-
         // Handle any intent
         handleIntent(intent)
     }
@@ -638,6 +634,11 @@ class MainActivity : AppCompatActivity(), Logging {
         unregisterMeshReceiver() // No point in receiving updates while the GUI is gone, we'll get them when the user launches the activity
         unbindMeshService()
 
+        scanModel.changeDeviceAddress.removeObservers(this)
+        model.connectionState.removeObservers(this)
+        bluetoothViewModel.enabled.removeObservers(this)
+        model.requestChannelUrl.removeObservers(this)
+
         super.onStop()
     }
 
@@ -658,6 +659,10 @@ class MainActivity : AppCompatActivity(), Logging {
             }
         }
 
+        model.connectionState.observe(this) { connected ->
+            updateConnectionStatusImage(connected)
+        }
+
         bluetoothViewModel.enabled.observe(this) { enabled ->
             if (!enabled && !requestedEnable && scanModel.selectedBluetooth) {
                 requestedEnable = true