Need to add a check for null for gutenberg_is_block_editor function

**Describe the bug**
A clear and concise description of what the bug is.
We are using a plugin that uses the wp_iframe method and in some way, `gutenberg_is_block_editor` from `lib/widgets.php on line 15` is being invoked. Since it doesn't check for null however, it breaks the functionality of the plugin since that iframe is triggering an error. Will push up a fix to check for null. 
