Cannot bind argument to parameter "FilePath" because it is null - powershell

Question

Every time I run the script below I get
Cannot bind argument to parameter 'FilePath' because it is null.
It was working last night. No changes have been made and this morning it just fails. the funny thing is if i save the script and then run it, it works. However if I clear console then run it again it fails.
Am I missing something obvious?
New-Item -ItemType Directory -Force -Path C:\NonStandard_Services
set-location C:\NonStandard_Services
$Computers= Get-Content C:\computers.txt
$Report= $file
$file= $Computer
ForEach ($Computer in $Computers)
{
Get-WmiObject -ComputerName $Computer -class Win32_Service -ErrorAction SilentlyContinue |
Where-Object -FilterScript {$_.StartName -ne "LocalSystem"}|
Where-Object -FilterScript {$_.StartName -ne "NT AUTHORITY\NetworkService"} |
Where-Object -FilterScript {$_.StartName -ne "NT AUTHORITY\LocalService"} |
Select-Object -Property StartName,Name,DisplayName|
ConvertTo-Html -Property StartName,Name,DisplayName -head $HTML -body "<H2>Non- Standard Service Accounts on $Computer</H2>"| Out-File $Report -Append}
#Rename-Item c:\GP_Services\Report.htm $file
Get-ChildItem | Where-Object {$_.extension -ne ".htm"} | Rename-Item -newname { $_.name + '.htm' }

Answer 1

$Report= $file
$file= $Computer
ForEach ($Computer in $Computers)
{
...
}
You assign variables to other variables before they were assigned a value themselves. Change the above to this:
ForEach ($Computer in $Computers) {
$file = $Computer
$Report = $file
...
}
Or directly use $Computer in Out-File:
... | Out-File "$Computer.txt" -Append