Invalid Menu Items

I’ve identified two issues, the first, this:

            'rewrite' => true,
            'rewrite' => array( 'slug' => 'districts' ),

Can just be this:

            'rewrite' => array( 'slug' => 'districts' ),

And the second, the reason you get invalid menu items, is because districts is being used as the rewrite slug, but, the internal namevof the taxonomy is Districts.

Replacing Districts with districts as the internal name fixed the issue for me locally:

register_taxonomy( 'districts', array('post'), array(

It’s a general rule of thumb to keep internal names of things lower case, you can always use the labels for what the user sees