[Bug]: better error message when trying to use nested maps of maps, maps of arrays, etc

[endel]

When user tries to define nested complex types, like this:

@type({ map: { map: "number" } })
playerScores = new MapSchema<MapSchema<number>>();

@type({ map: { array: "string" } })
playerTags = new MapSchema<ArraySchema<string>>();

The server crashes with the following error:

TypeError: Cannot read properties of null (reading 'Symbol(Symbol.metadata)')

This error is not very helpful. We need to make it more explicit this feature is not supported at the moment.