We try to get the value out of a dictionary in order to establish its
type for completion purposes. However, if the dictionary or the key
is not a constant, we cannot safely get the actual value, so we skip
this and just try to infer from static typing.
Getting the value directly with `Variant::get()` generate errors if the
base is a Dictionary and the key is of an invalid type. So before trying
to get it we use the Dictionary validator to make sure it we can safely
try to get the key.
