The problem with gui.cs is once you create something, you can't create another. I.e. you can't run this script twice.

FileEditor.Gui.ps1

using namespace Terminal.Gui # maybe it isn't installed yet, but who cares?
if (!(Test-Path Libraries)) {
    mkdir Libraries -Confirm
}
if (!(Test-Path .\Libraries\Terminal.Gui.0.24.0\lib\netcoreapp2.0\Terminal.Gui.dll)) {
    Install-Package Terminal.Gui -Destination .\Libraries\
}
add-type -path .\Libraries\NStack.Core.0.11.0\lib\netstandard1.5\NStack.dll
add-type -path .\Libraries\Terminal.Gui.0.24.0\lib\netcoreapp2.0\Terminal.Gui.dll

$Win = [Window]::new("Posh")
$global:Text = [TextView]@{
    Width    = [Dim]::Fill()
    Height   = [Dim]::Fill()
    ReadOnly = $false
}
$Win.Add($global:Text)

$Menu = [MenuBar]::new([MenuBarItem[]]@(
        [MenuBarItem]::new("_File", [MenuItem[]]@(
                [MenuItem]::new("_New", "Creates new file", {
                        $global:Text.Text = ""
                        $global:OpenFile = ""
                    }),
                [MenuItem]::new("_Save", "Save the file", {
                        if (!$global:OpenFile) {
                            $dialog = [SaveDialog]::new("Save", "Save as...")
                            [Application]::Run($dialog)
                            Write-Warning "FileName $($dialog.FileName)"
                            Write-Warning "FilePath $($dialog.FilePath)"
                            $global:OpenFile = [string]$dialog.FileName
                        }
                        if ($global:Text.Text) {
                            Set-Content $global:OpenFile $global:Text.Text
                        }
                        $global:Text.EnsureFocus()
                    }),
                [MenuItem]::new("Save _As", "Save with a new name", {
                        $dialog = [SaveDialog]::new("Save", "Save as...")
                        [Application]::Run($dialog)
                        $global:OpenFile = [string]$dialog.FileName
                        if ($global:Text.Text) {
                            Set-Content $global:OpenFile $global:Text.Text
                        }
                        $global:Text.EnsureFocus()
                    }),
                [MenuItem]::new("_Open", "Opens an existing file", {
                        $dialog = [OpenDialog]::new("Open", "Open a file")
                        [Application]::Run($dialog)
                        $global:OpenFile = [string]$dialog.FilePaths[0]
                        $global:Text.LoadFile($global:OpenFile)
                        $global:Text.EnsureFocus()
                    }),
                [MenuItem]::new("_Close", "", { }),
                [MenuItem]::new("E_xit", "", { [Application]::RequestStop() })
            ))
    ))

[Application]::Init()
[Application]::Top.Add($win, $menu);
[Application]::Run()