Vizit tries to keep all of its features in line, but sometimes things don't go as planned. In this case, the Vizit.Shared feature probably isn't installed.
Try running these two stsadm commands in the SharePoint Management Shell (remember to put in the right Site Collection path):
stsadm -o installfeature -name vizit.shared stsadm -o activatefeature -name vizit.shared -url http://server/site/path
or, if you'd prefer PowerShell (again, remember to put in the right Site Collection path):
Install-SPFeature -path "Vizit.Shared" Enable-SPFeature -Identity Vizit.Shared -Url http://server/site/path
Then recycle your IIS Application Pool, and you should be good to go!
Here are some links with more information on running stsadm: http://technet.microsoft.com/en-us/library/cc261956(v=office.12).aspx
or Windows PowerShell for SharePoint: http://technet.microsoft.com/en-us/library/ff607825.aspx
Note: Customers upgrading from versions prior to 5.0 may see a different error, but the same solution applies. The error "Object reference not set to the instance of an object" seen in Vizit Essential immediately after an upgrade would indicate that the Vizit.Shared feature is not installed and the above instructions should be followed.