PowerShell: Cannot index into a null array.

Cannot index into a null array. means the expression before [...] is $null. PowerShell lets you read $null and properties of $null without error, but indexing it fails. Find out why the variable is empty: usually a command that found nothing, or a typo in the variable or property name. Wrapping the result in @() makes it an empty array, so [0] returns $null instead of failing.

Reproduce it

$ErrorActionPreference = 'Stop'
$files = Get-ChildItem -Filter *.nomatch
try { $files[0] } catch { $_.Exception.Message; '  -> ' + $_.FullyQualifiedErrorId }
"@() result count: $(@(Get-ChildItem -Filter *.nomatch).Count)"
"first of @(): [$(@(Get-ChildItem -Filter *.nomatch)[0])]"
'a.txt', 'b.txt' | ForEach-Object { Set-Content $_ 'x' }
(Get-ChildItem -Filter *.txt | Sort-Object Name)[0].Name

Cannot index into a null array.
  -> NullArray
@() result count: 0
first of @(): []
a.txt

One result is not an array either

A command that returns one object returns the object itself, not a one-element array. $result[0] then indexes into that object (a string's first character, for example) instead of returning it. @() fixes both cases:

'only.log' | ForEach-Object { Set-Content $_ 'x' }
$one = Get-ChildItem -Filter *.log
"type without @(): $($one.GetType().Name)"
"type with @():    $(@(Get-ChildItem -Filter *.log).GetType().Name)"
"string index: $('hello'[0])"

type without @(): FileInfo
type with @():    Object[]
string index: h

Safer patterns