在PowerShell中将对象作为类成员创建一个数组

运行下面的代码时，我不断收到错误消息。我不明白我在做什么错

错误1：您不能在空值表达式上调用方法。在线：25字符：9

错误2：无法索引为空数组。在线：43字符：1

码：

            Class Step {
                [int]$StepNumber = 0
                [string]$Name = ''
                [string]$ScriptFile = ''
                [int]$status = 0 # 0 = 'InComplete', 1 = 'Complete', 2 = 'Failed', -1 = 'Skipped'
                [string]$DependencyStepNumber = -1

                Step([string]$Name, [int]$StepNumber, [string]$ScriptFile) {
                    $this.Name = $Name
                    $this.StepNumber = $StepNumber
                    $this.ScriptFile = $ScriptFile
                }
            }


            Class Guide {
                [int]$StepNumberCounter = 0
                [string]$Name = ''
                [Step[]]$Steps

                [int]AddStep([string]$Name, [string]$ScriptFile) {
                    $newStepNumber = $this.GetNewStepNumber()
                    $newStep = [Step]::new($Name, $newStepNumber, $ScriptFile)

line-25>>>          $this.Steps.Add($newStep)

                    return $newStepNumber
                }

                [int]GetStepName([int]$StepNumber) {
                    return $this.Steps[$StepNumber-1]
                }

                [int]GetNewStepNumber() {
                    return $this.StepNumberCounter += 1
                }
             }

             $Guide = [Guide]::new()
             $Guide.AddStep('Step One','ScriptOne.ps1')
             $Guide.AddStep('Step Two','ScriptTwo.ps1')

line-43>>>   Write-Host $Guide.Steps[0]
             Write-Host $Guide.Steps[1]