selectedItem property of Select$ChangeEventParameters should not be optional

[biscimus]

Describe the bug

public async onChangeDimensionType(event: sap.m.Select$ChangeEvent) {
        const selectedItem = event.getParameter("selectedItem"); 
}

TS tells me that selectedItem might be undefined, which is wrong.

Expected behavior
Should return a selected item, as defined by the UI5 API reference.

[nkappler]

This bothers me alot, too, as not only this interface is affected, but virtually every event parameter interfaces members are optional, even when there are always present in practice.

[akudev]

The control metadata has no way to express whether the parameter is always there (or is never null). In particular the Select doesn't specify anything:
https://github.com/UI5/openui5/blob/master/src/sap.m/src/sap/m/Select.js#L522-L524

There is a way to specify by manually adding JSDoc, as it is done here for example. This difference even shows up in the API documentation.

So... all good and just fix the dts-generator to use this information?

Unfortunately not: the generic/generated ones are interpreted as "mandatory" in api.json and hence in the API documentation, which is likely wrong in many cases, as this is just the default and developers have not explicitly specified that the parameter will be always there. Without TypeScript this was not really an issue, now it makes a difference:
Applying the same logic in the dts-generator would mean that many nullable/optional event parameters would be marked as mandatory. As result, the TypeScript compiler would gladly accept code expecting the value to never be null, but at runtime the code will crash in many cases. This is against the purpose of type checking, so we cannot solve it like this.

A cleaner solution would be to err on the safe side for all the generic cases where control developers have not explicitly specified whether the parameter can be null, BUT to identify those cases where additional JSDoc has been written. In those cases we can reasonably assume that the JSDoc is correct (or have it fixed if not).
This would require some adaptations here in dts-generator, plus a change in the JSDoc parser in the OpenUI5 repo, plus explicit JSDoc for the Select's "change" event. And in all others where mandatory event parameters are bothering people, which could be a lot... But this is the direction we are looking into right now.