fix(iOS): Add view check for getting StackView in goBackGesture (so…

…ftware-mansion#2060)

## Description

This PR fixes problem with goBackGesture screen transition. If stackView
can not be found, transition just shouldn't start instead of crashing
app and throwing the `Invalid view type, expecting RNSScreenStackView`
error.

## Checklist

- [x] Ensured that CI passes

ios/RNSModule.mm

@@ -115,7 +115,7 @@ - (void)disableSwipeBackForTopScreen:(NSNumber *)stackTag
 - (RNSScreenStackView *)getStackView:(NSNumber *)stackTag
 {
   RNSScreenStackView *view = [self getScreenStackView:stackTag];
-  if (![view isKindOfClass:[RNSScreenStackView class]]) {
+  if (view != nil && ![view isKindOfClass:[RNSScreenStackView class]]) {
     RCTLogError(@"Invalid view type, expecting RNSScreenStackView, got: %@", view);
     return nil;
   }