Add error handling for Automattic\WooCommerceBlocks\RestApi::get_routes_from_namespace().

[nerrad]

In various places in WordPress.org forums and at least one instance in our repo (see #4647) I've noticed reports of this error show up:

Fatal error: Uncaught Error: Call to undefined method WP_Error::get_data()

The callstack for that usually points to this line:

https://github.com/woocommerce/woocommerce-gutenberg-products-block/blob/trunk/src/RestApi.php#L64

While at least one report (#4647) is triggered in the context of Elementor being used on the site, I suspect the underlying cause is that the function in question gets called before the WP_REST_Server is setup and/or before the namespaced being called is registered.

At a minimum, we should add some WP_Error handling in this method and return null if it's present. This will prevent the fatal error from happening in those contexts and remove it from being a potential red herring for the real issue that may be present in this scenarios.

[github-actions]

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