Don't crash when saving with an image without an associated on-disk l…

…ocation

Fix #303
Reported by @jiyuuchc

src/main/java/fiji/plugin/trackmate/util/TMUtils.java

@@ -745,7 +745,7 @@ public static File proposeTrackMateSaveFile( final Settings settings, final Logg
 			 */
 			settings.imageFolder = settings.imp.getOriginalFileInfo().directory;
 		}
-		else if ( !settings.imageFolder.isEmpty() )
+		else if ( null != settings.imageFolder && !settings.imageFolder.isEmpty() )
 		{
 			final String absolutePath = FileSystems.getDefault().getPath( settings.imageFolder ).normalize().toAbsolutePath().toString();
 			folder = new File( absolutePath );