ConcurrentModificationException during rendering of Css/JsImport element.

In Css/JsImport.render(), the src/href attribute is temporarily removed:

// Temporarily remove href attribute while other attributes are rendered
setAttribute("href", null);

appendAttributes(buffer);

// Restore href attribute
setAttribute("href", href);

This cause incorrect rendering and ConcurrentModificationException when other threads rendering the same element. The sharing between threads is common for the CSS & JS elements added to a cached menu.

It is quite easy to reproduce: Just let jmeter hit localhost:8080/click-examples/home.htm about 40.000 times. After the test is finished use a browser to show the examples homepage. For me, the CSS and JS import elements related to menu is missing or rendered incorrectly.