better code (thanks @twpayne)

src/ol/base/createmap.js

@@ -134,16 +134,18 @@ ol.createMap = function(target, opt_values, opt_rendererHints) {
     if (rendererHint == ol.RendererHint.DOM) {
       if (ol.ENABLE_DOM && ol.dom.isSupported()) {
         rendererConstructor = ol.dom.MapRenderer;
-        return new ol.Map(target, rendererConstructor, values);
       }
     } else if (rendererHint == ol.RendererHint.WEBGL) {
       if (ol.ENABLE_WEBGL && ol.webgl.isSupported()) {
         rendererConstructor = ol.webgl.MapRenderer;
-        return new ol.Map(target, rendererConstructor, values);
       }
     }
   }
 
-  return null;
+  if (goog.isDef(rendererConstructor)) {
+    return new ol.Map(target, rendererConstructor, values);
+  } else {
+    return null;
+  }
 
 };