Export modal close button is invisible to screen readers

Steps to reproduce:

1. Log in and open any page (for example Main.WebHome).
2. Click the button "More Actions" top-right, then click Export to open the Export modal.
3. Turn on the screen reader (Narrator with Win+Ctrl+Enter, or NVDA) and press Tab through the modal.
4. Focus the X close button top-right and listen to the announcement.

Expected result:
The X is announced as "Close, button".

Actual result:

The X has no accessible name. Narrator announces "export dialog, button" instead of "Close, button".