[BUG] Crash when media download location prompts more than once

[geominorai]

Describe the bug

When the download handling is set to ask, the app will crash if you tap any media download button more than once, causing the file browser to open multiple times but crash the app when closing the last one.

This seems to be caused by the UI not blocking the download button from being pressed again while the file browser is still loading and yet to appear.

The crash happens regardless of whether a file name is chosen for the download or if the file browser is backed out. (Side note: Normally if you back out without choosing a file name, it does not crash but still shows a banner at the bottom saying it's downloading the media despite canceling.)

To Reproduce

Steps to reproduce the behavior:

1. Set download handling to always ask.
2. Open any image post.
3. Tap the download button multiple times in quick succession.
4. Close all file browser popups (one for each click of the download button).
5. See crash.

Device

Model: Samsung Galaxy S9+
OS: Android 10
Language: English
Fritter Version: v3.0.0-beta5