Gets the MainContext with which the source is associated.
You can call this on a source that has been destroyed, provided that the MainContext it was attached to still exists (in which case it will return that MainContext) . In particular, you can always call this function on the source returned from current_source. But calling this function on a source whose MainContext has been destroyed is an error.
the MainContext with which the source is associated, or null if the context has not yet been added to a source.