getValueWithKey
Looks up the TileHolder whose id equals tileId and asks it for the value it exposes under key, via that holder's own produceValueWithKey override.
Return
a single-entry map pairing key with the requested value, or null if no tile carries tileId, or that tile exposes nothing for key.
Parameters
tileId
id of the tile to read from.
key
the data key requested — meaning is entirely up to the target tile's own implementation (e.g. "text" for a TextField, or an implicit single key like Checkbox's checked).