quick fix of issue
Merge branch 'master' into Issue2638
add test cases for #2638
make status msg of nullPostParamMsg can display
catch msg for admin and the rest of cases
update testing for nullPostParamException
[2684] Status message not displayed, when handle NullPostParameterException in ControllerServlet.java
[2638]'preview as student' is not accessible to co-owner
Merge branch 'master' into Issue2587