fix(type): item in isItemDisabled may be undefined (fixes #1623)

[KrzysztofKarol]

What:

Typings changed. Item in isItemDisabled may be undefined, e. g.

downshift/src/hooks/utils.js

Line 659 in 59d3498

props.isItemDisabled(props.items[highlightedIndex], highlightedIndex)

Why:

Developers should handle this case to avoid runtime errors.

How:

Only typings changed.

Checklist:

• Documentation
• Tests
• TypeScript Types
• Flow Types
• Ready to be merged
  

Fixes #1623

Offtopic
Probably types of others methods should be check and fixed
e.g.

-itemToString?: (item: Item | null) => string
+itemToString?: (item: Item | undefined) => string

because

downshift/src/hooks/utils.js

Line 609 in 59d3498

inputValue: props.itemToString(props.items[highlightedIndex]),

[KrzysztofKarol]

Is there anything I can do to help?