fix imageviewer not showing up when using the actions menu

apps/files_imageviewer/js/lightbox.js

@@ -1,3 +1,4 @@
+var lightBoxShown=false;
 $(document).ready(function() {
 	images={};//image cache
 	FileActions.register('image','View',function(filename){
@@ -38,9 +39,12 @@ function showLightbox(container,img){
 	container.append(img);
 	container.css('top',Math.round( ($( window ).height() - img.height)/2));
 	container.css('left',Math.round( ($( window ).width() - img.width)/2));
+	lightBoxShown=true;
 }
 
 function hideLightbox(){
+	if(lightBoxShown){
 		$('#lightbox_overlay').remove();
 		$('#lightbox').remove();
+	}
 }