Fix crash in Session Detail when Activity recreated

[panpanini]

Issue

• close #542

Overview (Required)

• Allows null + quick return for speakers list
• attempts to simplify on click listener behavior

I'm not sure if this is the right place to simplify the click listener, so I can remove it and create another PR if its better :bow:

[takahirom]

I think it is ok. Thanks 👍 LGTM 👍

[panpanini]

Done! sorry 🙏