Avoid Null reference exceptions in service discovery callbacks

Calling disconnect during service discovery often triggers null ref exceptions, sometimes crashing the app.

src/android/Peripheral.java

@@ -377,7 +377,9 @@ public void onServicesDiscovered(BluetoothGatt gatt, int status) {
             if (refreshCallback != null) {
                 refreshCallback.sendPluginResult(result);
                 refreshCallback = null;
-            } else {
+            }
+
+            if (connectCallback != null) {
                 connectCallback.sendPluginResult(result);
             }
         } else {