What happens when one part of my Will is invalid? Does it make the whole Will null and void?

It really depends what exactly is the part that went wrong. Say if there's only 1 witness signing instead of 2, then entire Will is invalid to begin with. 

Or, say you wrote something in your Will that can't be done. For example, you wrote in your Will to gift a jointly owned property to anyone other than the other joint tenant. In this particular case, that clause is invalid, but the other sections of your Will distributing your other assets are still valid.

Your Will states your intention on how you want your assets to be distributed after your death, which will be enforced as much as possible according to it.