Fix NullPointerException in NavigationModule

[Wiingaard]

We're seeing a crash that affects ~1% of our user base per month, that's caused by:

NullPointerException
Attempt to invoke virtual method 'boolean android.app.Activity.isFinishing()' on a null object reference

After looking into the code, we realised that that the null check and .isFinishing() are not referencing the same activity.

Here's a screenshot with stack trace from Sentry:

[stale[bot]]

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