Powershell script to open forms in dialog

I have written some Powershell script to iterate through each subsite in a web application, the aim of the script is then to find a list in each subsite with a specific name.

For example, our lists are called "Team Messages" and my aim is to turn on the open forms in a dialog.

The script I have is:

 Add-PSSnapin "Microsoft.SharePoint.PowerShell"

# Get All Web Applications
$WebApps=Get-SPWebApplication
foreach($webApp in $WebApps)
{
    foreach ($SPsite in $webApp.Sites)
    {
       # get the collection of webs
       foreach($SPweb in $SPsite.AllWebs)
        {
            write-host $SPweb.title
               foreach($list in $web.Lists["Team Messages"])
               {
                    $list.NavigateForFormsPages = $false;               
                    $list.Update();                 
               }
        }
    }
  }

The code runs up to the final foreach loop where it then throws an error that is

Cannot index into a null array.

If anyone could point me in the right direction with this then it would be great!