call ON_FRAGMENT_CREATED before INIT_PAYMENTSHEET to prevent potential null pointer crashes

[remonh87]

We noticed that we get from the Flutter Stripe community some nullpinter exceptions that happen a bit randomly. See issue flutter-stripe/flutter_stripe#416 . In my current project I also saw this issue happen and it looks like a race condition that causes the following cast being executed in the StripeModule while the fragment is being null.

if (intent.action == ON_FRAGMENT_CREATED) {
        paymentSheetFragment = (currentActivity as AppCompatActivity).supportFragmentManager.findFragmentByTag("payment_sheet_launch_fragment") as PaymentSheetFragment
      }

After some investigation it looks like the promise is being resolved before the fragment is created. This fix will prevent this from happening.

[CLAassistant]

All committers have signed the CLA.

[charliecruzan-stripe]

Since intents are asynchronous, I think it's still possible to run into this issue even afterwards. What do you think of removing the need for the ON_FRAGMENT_CREATED intent altogether, like this: #788?

[remonh87]

Since intents are asynchronous, I think it's still possible to run into this issue even afterwards. What do you think of removing the need for the ON_FRAGMENT_CREATED intent altogether, like this: #788?

@charliecruzan-stripe yes that can work. Let's go for your solution I will close this one