Edit Upload Document

A change needs to be made in the example code in the Upload document:
https://vaadin.com/docs/-/part/framework/components/components-upload.html

try {
    // Open the file for writing.
    file = new File("/tmp/uploads/" + filename);
    fos = new FileOutputStream(file);
} catch (final java.io.FileNotFoundException e) {
    new Notification("Could not open file<br/>", e.getMessage(), Notification.Type.ERROR_MESSAGE) .show(Page.getCurrent());
    return null;  // <--- BAD
}

Returning null throws the following exception com.vaadin.server.NoOutputStreamException.  The fix is to instead return NullOutputStream().

import org.apache.commons.io.output.NullOutputStream;

try {
    // Open the file for writing.
    file = new File("/tmp/uploads/" + filename); 
    fos = new FileOutputStream(file); 
} catch (final java.io.FileNotFoundException e) { 
    new Notification("Could not open file<br/>", e.getMessage(), Notification.Type.ERROR_MESSAGE) .show(Page.getCurrent()); 
    return NullOutputStream();  // <--- GOOD
}

Furthermore, it might be useful to show an example of how the method might handle incorrect files:

if (!filename.matches("(?i).+\\.(jpg|gif|png)$")) {
    Notification.show("Must be a .jpg, .gif or .png");
    return NullOutputStream();
}

try {
    ...
} catch {
    ...
}