Exception when opening script if the last directory isn't available

[petebankhead]

Bug report

Describe the bug
See https://forum.image.sc/t/qupath-script-editor-error-when-trying-to-open-saved-script/89826/1

To Reproduce
Steps to reproduce the behavior:

1. Open a script in the script editor, with File → Open
2. Delete or rename the folder containing the script
3. Try to open a script in the script editor, with File → Open
4. See the exception

18:07:12.434	[JavaFX Application Thread]	ERROR	qupath.lib.gui.QuPathUncaughtExceptionHandler	Folder parameter must be a valid folder	java.lang.IllegalArgumentException: Folder parameter must be a valid folder
	at com.sun.glass.ui.CommonDialogs.convertFolder(CommonDialogs.java:239)
	at com.sun.glass.ui.CommonDialogs.showFileChooser(CommonDialogs.java:191)
	at com.sun.javafx.tk.quantum.QuantumToolkit.showFileChooser(QuantumToolkit.java:1719)
	at javafx.stage.FileChooser.showDialog(FileChooser.java:419)
	at javafx.stage.FileChooser.showOpenDialog(FileChooser.java:354)

Expected behavior
The open dialog opens properly.

Desktop (please complete the following information):

• OS: All (presumably, definitely macOS)
• QuPath Version: 0.5.0

Additional context
This happens when the previous directory used in the dialog doesn't exist. Other commands may be affected as well.

[petebankhead]

The fix may be best added in qupath-fxtras at https://github.com/qupath/qupath-fxtras/blob/main/src/main/java/qupath/fx/dialogs/FileChoosers.java#L629 (and probably for the directory chooser).