Aded empty selected directory fix

src/main/java/gui/CorpusTab.java

@@ -332,7 +332,11 @@ public class CorpusTab {
 	 * has to produce a non-empty list results list
 	 */
 	private void chooseCorpusLocation() {
+		File oldSelectedDirectory = selectedDirectory != null ? new File(selectedDirectory.getAbsolutePath()) : null;
 		selectedDirectory = directoryChooser();
+		if(selectedDirectory == null) {
+			selectedDirectory = oldSelectedDirectory;
+		}
 
 		if (selectedDirectory != null && ValidationUtil.isReadableDirectory(selectedDirectory)) {
 			logger.info("selected corpus dir: ", selectedDirectory.getAbsolutePath());