Fix error on RN 0.60

[mzdunek93]

When trying to render a CollapseView I get this error:

Invariant Violation: [659,"RCTView",311,{"height":"<<NaN>>","backgroundColor":0,"overflow":"hidden"}] is not usable as a native method argument

This is caused by giving an uninitiated Animated.Value as the component's height. I fixed it by setting the height as null until header and content layouts are measured.