The error you are seeing is happening because you are trying to access the MeetingService too soon. The SDK must be successfully initialized before any of the classes provided by it are non-null. You can find more information on how to initialize the SDK here.
Thanks for providing the code snippets. Just calling the initialize method on the SDK does not mean that it has been successfully initialized. In your ZoomSDKInitializeListener instance, you will need to receive a result with an error code of ZoomError.ZOOM_ERROR_SUCCESS. Upon receiving this, you can begin using the SDK to join or start a meeting.
Can you please tell me how to do this, or maybe give a link where I can find some information about this? I am not able to understand how to get a result from the instance.
Our SDK initialization documentation should provide you with all of the information you need to properly initialize the SDK. If you have any specific questions on this flow, please let me know and I will be happy to help.