java.lang.String java.lang.CharSequence.toString()' on a null object reference ThemeSwitcher

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**Android API: 29**
**Mapbox Navigation SDK version:5.7.0**

### Steps to trigger behavior

1. init navigation turn by turn navigation

### Expected behavior

### Actual behavior

java.lang.NullPointerException: Attempt to invoke interface method 'java.lang.String java.lang.CharSequence.toString()' on a null object reference
at com.mapbox.navigation.ui.internal.ThemeSwitcher.retrieveMapStyle(ThemeSwitcher.java:97)
at com.mapbox.navigation.ui.NavigationView.onMapReady(NavigationView.java:239)
at com.mapbox.mapboxsdk.maps.MapView$MapCallback.onMapReady(MapView.java:1320)
at com.mapbox.mapboxsdk.maps.MapView$MapCallback.initialised(MapView.java:1306)
at com.mapbox.mapboxsdk.maps.MapView.initialiseMap(MapView.java:197)
at com.mapbox.mapboxsdk.maps.MapView.access$800(MapView.java:63)
at com.mapbox.mapboxsdk.maps.MapView$6.run(MapView.java:353)
at android.os.Handler.handleCallback(Handler.java:790)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6626)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:811)

Start in ThemeSwitcher.java at line 97, where retrieveMapStyle is called during NavigationView.onMapReady. Reproduce the crash by initializing turn-by-turn navigation on Android API 29 with Mapbox Navigation SDK 5.7.0, then trace the null CharSequence. Done means initialization no longer throws this NullPointerException.

Written by the indexing model from the issue text.