Update advanced-dragndrop.asciidoc (#10680)

* Update advanced-dragndrop.asciidoc

Fixing example of Drag and drop code.
Resolves https://github.com/vaadin/framework/issues/10512

* Update advanced-dragndrop.asciidoc

Removed unnecessary spaces

* Update advanced-dragndrop.asciidoc

Removed unnecessary spaces

documentation/advanced/advanced-dragndrop.asciidoc

 Label dropArea = new Label("Drop files here");
 FileDropTarget<Label> dropTarget = new FileDropTarget<>(dropArea, event -> {
 
-    List<Html5File> files = event.getFiles();
+    Collection<Html5File> files = event.getFiles();
     files.forEach(file -> {
         // Max 1 MB files are uploaded
         if (file.getFileSize() <= 1024 * 1024) {
                 // Output stream to write the file to
                 @Override
                 public OutputStream getOutputStream() {
-                    return new FileOutputStream("/path/to/files/"
-                        + file.getFileName());
+                    try{
+                        return new FileOutputStream("/path/to/files/"
+                          + file.getFileName());
+                        }catch (FileNotFoundException e) {
+                                e.printStackTrace();
+                            }
+                    return null;
                 }
 
                 // Returns whether onProgress() is called during upload
                     Notification.show("Stream failed, fileName="
                         + event.getFileName());
                 }
+
+                @Override
+                public boolean isInterrupted() {
+                    return false;
+                }
             });
         }
     }