Preventing NullPointerException when calling Via.getManager().addEnableListener() after manager is initialized

[wtlgo]

If an external plugin calls Via.getManager().addEnableListener() after Via.getManager().init() has been called, it causes a NullPointerException. In this pull request, to prevent this issue, if the manager is already initialized, it immediately executes the listener instead of raising an exception and causing the external plugin to fail during its loading process.

[kennytv]

What's the use case here? If any of our plugins call this too late, this should mean failure (though ofc the exception could be better), also just being internal api

[wtlgo]

What's the use case here?

E.g. If the external plugin is loaded by another plugin during the loading phase via let's say Bukkit.getPluginManager().loadPlugin() it is quite literally impossible to manage the external plugin to call addEnableListener before ViaVersion calls init even though it is not misconfiguration, as all it tries to do is not to run its ViaVersion-dependent init process before ViaVersion is ready, and as result both ViaVersion and plugin in question would function just fine.

Also, this is not "just internal API", as it is exposed by com.viaversion:viaversion-api.

[kennytv]

Is that external plugin one of ours, or one you can change the method call in? The current name doesn't really imply any immediate execution and even in your scenario should still provide a warning. Assuming this is about ViaBackwards or Rewind, this will lead to peformance issues, possibly even more so in the future, due to missing data availability on load

[FlorianMichael]

Maybe we could consider moving this into common since it's not really API other plugins should use?