In order to have the Play on Realms button, you must have a server or be invited to a Realms server.
I confirm this as an issue.
@Blah I confirm that in MC-64829 Allocating 2GB of ram solves the issue.
Try Alt + F4 . Post the report in notepad, save it, and attatch it here.
Roof
In order to have the Play on Realms button, you must have a server or be invited to a Realms server.