Uncaught TypeError: Cannot read property 'offsetHeight' of null

I am getting below errors while trying to display zul files, this error occurs on allmost all browsers> **TypeError: Cannot read property 'offsetHeight' of null**>> **Failed to mount: Cannot read property 'offsetHeight' of null**>> TypeError: ao is null >>in **zk.wpd script file** Most of the times it works properly but only sometimes i get this error, i am not able to determine what is the root cause for it, will really appriciate if someone can point me to right direction to solve this issue.