should use $WhatIfPreference rather than $WhatIf

[WiredSharp]

robocopyps\0.2.13\functions\Invoke-RoboCopy.ps1:516 char:49

•     elseif ($Force -eq $True -and $null -eq $WhatIf) {
  

•                                             ~~~~~~~
  

• CategoryInfo : InvalidOperation: (WhatIf:String) [], RuntimeException
• FullyQualifiedErrorId : VariableIsUndefined