fix: add check to avoid crashes when currentActivity is null

[charliecruzan-stripe]

Summary

We weren't always appropriately null-checking currentActivity, and the typecast could result in crashes. We now resolve with a consistent and useful error telling the user they can retry the method.

Motivation

closes #844 and I'm sure other instances of crashes on Android
closes #881

Testing

• I tested this manually
• I added automated tests

Documentation

Select one:

• I have added relevant documentation for my changes.
• This PR does not result in any developer-facing changes.

[michelleb-stripe]

I was really surprised that you have to check the current activity for null everywhere, even from @ReactMethod, but I didn't really see another way.

In order to prevent this type of error in the future I would recommend:

class StripeSdkModule ...{
...
  // Set currentActivity so it uses this instead of the base class `getCurrentActivity()`
  // this will give you type safety checking and convert it to an appCompat activity
  val currentActivity: AppCompatActivity?
    get() = (super.getCurrentActivity() as? AppCompatActivity)
...
    private val googlePayReceiver: BroadcastReceiver = object : BroadcastReceiver() {
        override fun onReceive(context: Context?, intent: Intent) {
          if (intent.action == ON_GOOGLE_PAY_FRAGMENT_CREATED) {
            currentActivity?.let {
...
}

It would also be nice to see some enforcement in the code that if you can't get the appCompat activity that a promise is fired to return a result.

[charliecruzan-stripe]

thanks for the feedback! created a helper method for getting the activity or resolving the promise. It's not the perfect solution since someone can still call currentActivity, but it does avoid a lot of repeated code and hopefully anyone working here would see the current pattern for getting the activity 🤞

[charliecruzan-stripe]

@michelleb-stripe do you approve the most recent changes? I'll need to rebase but since #879 contains substantial changes to the same file, i'd rather wait till after that lands to rebase (to avoid duplicating the work)

[charliecruzan-stripe]

closes #881