[SalesForce] the best way to add comment in Visualforce Code

This is not an issue in your code. I usually get a similar Null Pointer Exception error that you got.

Error: Java.lang.NullPointerException
Error: null

I tried saving the page again after a couple of minutes and it got saved.

This might work for you too.

the reason for the error is the action attribute has to look like {!xxx}.

If you have an action method in your controller extension called addToCampaign(), then change the commandbutton to read like:

<apex:pageBlockButtons>
  <apex:commandButton action="{!addToCampaign}" value ="Add to a Campaign"/>
</apex:pageBlockButtons>

If you aren't overriding the addToCampaign action in your controller, then use

<apex:pageBlockButtons>
  <apex:commandButton action="{!URLFOR($Action.Contact.AddtoCampaign,Contact.id,[retURL=$CurrentPage.URL])}" 
                      value ="Add to a Campaign"/>
</apex:pageBlockButtons>