Increase version of application.config automatically using powershell after every deployment

I am new to powershell and I wanted to change version in an xml file after every deployment and. I referred to Unable to update a value in msbuild proj file using powershell. Below is the xml content:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <site>


      <add key="ShowAppConf" value="true"/>
      <add key="site.IsCsrfCheck" value="false" />
      <add key="EnableDeviceFileAuthentication" value="false" />
      <add key="EnableFileAuthentication" value="false" />
      <add key="AppVersion" value="v0.1.7.21.31.144402" />


    </site>
</configuration>  

and I am trying to increase value of revision version of AppVersion by using:

$Test1QAMSBuildFile = 'D:\test\application.config.xml'
[xml]$Test1QABuildVersion = Get-Content $Test1QAMSBuildFile
$node = $Test1QABuildVersion.SelectSingleNode("/configuration/site/key/AppVersion")
$PropertyVersion= $node.InnerText
$UpdateVersion= $PropertyVersion.split(".")
$UpdateVersion[4] = (($UpdateVersion[3] -as [int]) + 1).ToString()
$newVersion = $UpdateVersion -join '.'
$node.InnerText = $newVersion
$Test1QABuildVersion.Save($Test1QAMSBuildFile)

After running this script powershell ise throws error:

You cannot call a method on a null-valued expression.
At line:5 char:1
+ $UpdateVersion= $PropertyVersion.split(".")
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull

Cannot index into a null array.
At line:6 char:1
+ $UpdateVersion[4] = (($UpdateVersion[3] -as [int]) + 1).ToString()
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
    + FullyQualifiedErrorId : NullArray

The property 'InnerText' cannot be found on this object. Verify that the property exists and can be set.
At line:8 char:1
+ $node.InnerText = $newVersion
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
    + FullyQualifiedErrorId : PropertyNotFound

How do I increase version automatically using powershell.