fix: Empty array & null checks in handleTabNavigatorChange method fixes #6698

[aymather]

Fixes: Shared Element Transition (Solution provided) #6698

Add safety checks in handleTabNavigatorChange to prevent crash

This fixes a crash that occurs when _disappearingScreens array is empty or contains invalid objects by adding appropriate null checks and bounds checking.

The crash would occur when:

• The _disappearingScreens array is empty
• navTabScreen or its reactSuperview is null
• targetScreen is null

App was crashing during shared element transition because of invalid array access.

Fixes #6698

[aymather]

I fixed one of the errors, but I'm not quite sure how exactly to fix the other failing check? Can I get some guidance?