Should be replaced with assert, because it can be null only for code with syntax errors.

Common rule: if some method of PSI (``JetExpression``, ``JetWhenCondition``, ``JetWhatever``...) is annotated as ``@Nullable @IfNotParsed``, it returns null only if code has syntax errors. Front-end should be tolerant to nulls here, while back-end can be assert it.