Resolve "New project form errors out on Cell 2"

What does this MR do and why?

Fix is based on the comment at #427660 (comment 1636407970)

The error described in #427660 (closed), happens due to the fact that the current_user does not have a personal namespace on cell-2. (Namespaces are cell-local, and currently a user's personal namespace is still on cell-1 and we do not plan on replicating it across cells)

So, on cell-2, current_user.namespace.id would naturally fail with error, because current_user.namespace would return nil

To prevent this from happening, we use current_user.namespace_id instead, which is already defined in the model as

  def namespace_id
    namespace.try :id
  end

So, on cell-2, this method will gracefully return nil without error, thus fixing our problem. On cell 1, it will continue to function as before.

Screenshots or screen recordings

After the fix:

On cell-1	On cell-2

On cell-2, we can see that no "Users" section is shown because it is unavailable, and the page does not error out.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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Related to #427660 (closed)