When reporting an error, it is better to point on which line it happens. Here, we can easily run your code, but that's not always the case.
But no need to run your code, your error is a classical one: you declare a global rectangle array (better use plural for arrays and collections, BTW), then you re-declare a local one in setup(), which shadows the global one, and disappear at the end of setup(). The global one remains uninitialized, hence the error.
Solution:
Either
- do the declaration and the initialization in the same line, and the global level;
- or remove Rectangle[] inside setup().

Hi!