fix: guard null promise in case of skipCodeExchange (#595)

Co-authored-by: Fallen Destiny <[email protected]>

Author: FallenDestiny

android/src/main/java/com/rnappauth/RNAppAuthModule.java

@@ -397,7 +397,9 @@ public void onActivityResult(Activity activity, int requestCode, int resultCode,
 
             if (this.skipCodeExchange) {
                 WritableMap map = TokenResponseFactory.authorizationResponseToMap(response);
-                promise.resolve(map);
+                if (promise != null) {
+                    promise.resolve(map);
+                }
                 return;
             }