Fix WSOD on subsites when creating subsite page in front end theme

[andybroomfield]

Fix #154

What does this change?

Checks the entity ID is numeric (is not null) when trying to find subsite.
Adds a test to verify status code is 200 (500 without this fix).

How to test

1. Set appearance to use the front end theme as the admin theme, or use an editor like mercury editor.
2. Create a new Subsite page at /node/add/localgov_subsite_page
3. Should be able to add a page without errors.

How can we measure success?

Creating a subsite page no longer crashes.

Have we considered potential risks?

Need to check side effects on the blocks not finding their subsite.
I noticed that using is_int does fail as $entity->id() brings back a string, might fail strict types.

Images

N/A - Using front end theme is optional for sites that want to do so.

Accessibility

N/A

[millnut]

Would changing to is_null work if it could be a string?

Suggested change

	&& is_numeric($entity->id())
	&& !is_null($entity->id())

[andybroomfield]

We can try that, if that would be more correct.

[stephen-cox]

I haven't checked this fixes creating a node through the a frontend editor, but the change fixes what the test checks and it doesn't look to have any other side-effects.