Update Master Page for apps

some times you can face issues with apps due to change in master pages

The error happens when the master page of the parent site is applied to the app web  of the Sharepoint App.

In my case I could reproduce this by reapplying the master page in the UI and selecting the  “Reset all subsites to inherit the master page option”. 

# if no arguments added display error and exit
if ($args.Length -eq 0)
{
Write-Host -ForegroundColor Red "Missing Required Argument"
Write-Host -ForegroundColor Red "Usage - 'ResetAppMasterPages <<site collection url>>' "
Exit 0
}

$siteCollUrl =$args[0]

Add-PSSnapin Microsoft.SharePoint.PowerShell -ERRORAction SilentlyContinue | Out-Null
#start logging
$LogFile = "$($FarmConfig.Farm.ScriptConfig.ScriptLoggingFolder)$($MyInvocation.MyCommand.Name.Replace('.ps1',''))-$(Get-Date -Format yyyy-MM-dd_h-mm-ss).txt"
Start-Transcript -Path $LogFile -Force
Start-SPAssignment -Global

$site = $null
try
{
$site = Get-SPSite $siteCollUrl

#iterate through all the webs
foreach ($web in $site.AllWebs)
{
#check if the web is an app web
if ($web.IsAppWeb -eq $true)
{
try
{
Write-Host "Updating master page for : "  $web.Url

#Below lines will help to keep history of old master page urls in the log
Write-Host "Old MasterUrl : " $web.MasterUrl
Write-Host "Old CustomMasterUrl : " $web.CustomMasterUrl

#update master page to app.master
$web.CustomMasterUrl = $web.ServerRelativeUrl + "/_catalogs/masterpage/app.master"
$web.MasterUrl = $web.ServerRelativeUrl + "/_catalogs/masterpage/app.master"
$web.Update()

Write-Host -ForegroundColor Green "Successfully updated master page for : "  $web.Url
}
catch
{$_}
finally
{
if ($null -ne $web)
{
# need to dispose web from allwebs property
$web.Dispose()
}
}
}
}
}
catch
{$_}
finally
{
if ($null -ne $site)
{
# dispose site object
$site.Dispose()
}
}

Stop-SPAssignment -Global
Stop-transcript