Fix drag and drop of SwingFileWidget

On Windows, trying to get the TransferData during canImport() throws an InvalidDnDOperationException.
Since we know the exception is coming, let's catch and ignore it.

See: https://coderanch.com/t/664525/java/Invalid-Drag-Drop-Exception

Thanks Stefan Helfrich for investigating.

Author: imagejan

src/main/java/org/scijava/ui/swing/widget/SwingFileWidget.java

@@ -32,6 +32,7 @@
 
 import java.awt.datatransfer.DataFlavor;
 import java.awt.datatransfer.UnsupportedFlavorException;
+import java.awt.dnd.InvalidDnDOperationException;
 import java.awt.event.ActionEvent;
 import java.awt.event.ActionListener;
 import java.io.File;
@@ -314,12 +315,34 @@ public FileTransferHandler(final String style) {
 		@Override
 		public boolean canImport(final TransferHandler.TransferSupport support) {
 			if (!hasFiles(support)) return false;
-			final List<File> allFiles = getFiles(support);
-			if (allFiles == null || allFiles.size() != 1) return false;
 
-			final FileFilter filter = SwingFileWidget.createFileFilter(style);
-			final List<File> files = SwingFileWidget.filterFiles(allFiles, filter);
-			return files.size() == 1;
+			// We wish to test the content of the transfer data and
+			// determine if they are (a) files and (b) files we are
+			// actually interested in processing. So we need to call
+			// getTransferData() so that we can inspect the file names.
+			// Unfortunately, this will not always work.
+			//    Under Windows, the Transferable instance
+			// will have transfer data ONLY while the mouse button is
+			// depressed. However, when the user releases the mouse
+			// button, this method will be called one last time. And when
+			// when this method attempts to getTransferData, Java will throw
+			// an InvalidDnDOperationException. Since we know that the
+			// exception is coming, we simply catch it and ignore it.
+			// See:
+			// https://coderanch.com/t/664525/java/Invalid-Drag-Drop-Exception
+			try {
+				final List<File> allFiles = getFiles(support);
+				if (allFiles == null || allFiles.size() != 1)
+					return false;
+
+				final FileFilter filter = SwingFileWidget
+						.createFileFilter(style);
+				final List<File> files = SwingFileWidget.filterFiles(allFiles,
+						filter);
+				return files.size() == 1;
+			} catch (InvalidDnDOperationException exc) {
+				return true;
+			}
 		}
 
 		@Override