adjust default value for createContext example

[Kageetai]

passing in undefined here throws the following error:

TS2345: Argument of type 'undefined' is not assignable to parameter of type 'App'.

I propose this type cast instead, even though not the most elegant solution, it avoids the error, as the context is already set in initialization of the React context.

[Acylation]

I came across the same problem and would suggest another change here.
export const AppContext = React.createContext<App | undefined>(undefined); also works, please take it into consideration.

[marcusolsson]

If export const AppContext = React.createContext<App | undefined>(undefined); works, I think it's a clearer alternative.

[Kageetai]

Yeah, those suggestions make sense as well, but then when actually using the useApp hook, App can also be undefined (as defined for the hook) and has to be checked for existence. This line then throws a TS error for me.
Reminds me that for my project, I actually also removed the App | undefined from the definition of useApp, as it is implicitly derived.

Therefore with the code as currently here and the suggestions above, it would actually throw even two errors for me.