A closed and floating Timeline panel on start is black when opened

Shotcut crashs when open the Timeline panel being black and try to drag it.

Well I got a pretty indescribable issue.

OS: Linux Mint 20.1 Cinnamon
Graphic: Intel Corporation UHD Graphics 620 (Whiskey Lake) (prog-if 00 [VGA controller])
Shotcut versions: shotcut-linux-x86_64-210321.AppImage

Reproduce:

1. Click the square botton on the left of close botton of Timeline panel(I don't know what this button is called, maybu I would call it dock-button), to leave the Timeline panel undocked.
2. Click close botton of Timeline panel. The Timeline panel is closed. Also close the keyframe panel so it won't disturb.
3. Close Shotcut.
4. Reopen Shotcut. Now the Shotcut should open without Timeline panel. (Make the window unmaximized would be also make it easier)
5. Click View and click Timeline(or just press Ctrl+5) to reopen Timeline panel.
   What I wish: A functional Timeline panel, undocked.
   What I got: A Timeline panel filled with black, undocked.

6. Try to resize the Timeline panel. Shotcut would crash after this.

Temporary fix:
When the black timeline panel shows up, just close this panel and reopen it, or press the dock-button to dock it.

I think this is definitely a bug. It may crash Shotcut althrough it is hard to get triggered.

I also tried to start the appImage with cli to see if there is somehing useful but sadly there is only a "Segmentation fault (core dumped)" after the crash.

Start by reproducing the undocked Timeline-panel sequence in the issue on Linux Mint 20.1 using shotcut-linux-x86_64-210321.AppImage, then inspect the resulting segmentation fault when resizing the reopened panel. Done means the undocked Timeline displays normally after restarting and resizing it no longer crashes Shotcut.