This error will not prevent viewing the wiki content. It is an E_NOTICE error. You must have debug mode turned on. Since debug mode halts page execution, it can cause data to be saved in an incomplete form and create more unusual errors, such as the one you see now. You should only use debug mode in development.
The only way for this to occur is if one of the categories attached to this page has no revision history for its category-page. This is not normal. It would be impossible to access that category now. If you edit this page, you will probably see a strange category attached to it called "Wiki", which should be removed.
Was this category created and/or added to the page in a recent version? Do you remember what you were doing that achieved the broken category?
It is possible that it was created in an earlier version if you created the category or purged its current revision while XenForo's debug mode was turned on and encountered an E_NOTICE or other low-level error during creation. If the original E_NOTICE was resolved, then this report is really a duplicate. The error here is a side effect of maintaining the garbage category that was created.
Since the category has no name data, finding it can be tricky. If you rebuild URLs via the wiki's Maintenance tools, you might then be able to find and remove the category in URL Conflicts (it would probably be a collision with "Wiki").