$post not defined in gutenberg_menu()

[mrmadhat]

Describe the bug
gutenberg_menu() calls gutenberg_can_edit_post() with $post as an argument but it is not defined at that point.

To Reproduce
Steps to reproduce the behavior:

1. Load the wp-admin or single post
2. See error

Expected behavior
Check for the existing of $post before using it or using global $post so $post will at least be defined even if it's value is NULL

Screenshots
(added die() after call to gutenberg_can_edit_post() so error can be seen)

Desktop (please complete the following information):

• OS: IOS
• Browser Chrome

Additional context

• Master