The RemovableDriveOutputDevice was selecting the preferred format to save the slice output in automatically. To select this, it was taking the intersection between the available output formats (for which there is a Writer plug-in) and the file formats that the printer supports. However if the printer supports a file format that is not supported by Cura, it would crash here because it was looking up the plug-in for a MIME type it doesn't know about. Stupid mistake. But not half as stupid as the bug I'm about to fix in the next commit.
Fixes #4827.
