null check progress DOM element even if it's configured to be on

Author: hakimel

README.md

@@ -176,6 +176,7 @@ You can change the appearance of the speaker notes by editing the file at `plugi
 - Added beige theme
 - Added 'autoSlide' config
 - Bug fix: The 'slidechanged' event is now firing upon 'hashchange'. Thanks [basecode](https://github.com/basecode)
+- Bug fix: JS error when the 'progress' option was true but there was no progress DOM element
 
 #### 1.3
 - Revised keyboard shortcuts, including ESC for overview, N for next, P for previous. Thanks [mahemoff](https://github.com/mahemoff)

js/reveal.js

@@ -155,7 +155,7 @@ var Reveal = (function(){
 			dom.controls.style.display = 'block';
 		}
 
-		if( config.progress ) {
+		if( config.progress && dom.progress ) {
 			dom.progress.style.display = 'block';
 		}
 
@@ -680,7 +680,7 @@ var Reveal = (function(){
 		}
 
 		// Update progress if enabled
-		if( config.progress ) {
+		if( config.progress && dom.progress ) {
 			dom.progressbar.style.width = ( indexh / ( document.querySelectorAll( HORIZONTAL_SLIDES_SELECTOR ).length - 1 ) ) * window.innerWidth + 'px';
 		}