Add validation checks for help pages

**The Issue**
There is no vaildation checks for invalid help pages, which can cause NPE's when an invalid page is specified.

e.g. /help 15 will cause an NPE.

**Justification for this PR:**
To solve the issue listed above.

**PR Breakdown**
Checks if the requested pages are null, then either denies the action or lists the page, instead of throwing a NPE.