Error during handling of output-element within Write-NUnit3OutputElement

Checklist

[X] Issue has a meaningful title
[X] I have searched the existing issues. See all issues
[X] I have tested using the latest version of Pester. See Installation and update guide.

What is the issue?

Depending on the type of output the processing might fail during calling the ToString() method within Write-NUnit3OutputElement. As soon as this error occurs, the test result file (TestResult.OutputPath) is no longer written completely.

function Write-NUnit3OutputElement ($Output, [System.Xml.XmlWriter] $XmlWriter) {
    $outputString = @(foreach ($o in $Output) { $o.ToString() }) -join [System.Environment]::NewLine

    $XmlWriter.WriteStartElement('output')
    $XmlWriter.WriteCData($outputString)
    $XmlWriter.WriteEndElement()
}

Stacktrace might be

System.Management.Automation.RuntimeException: Invoking step End failed:
Result 1 - Error 1:Export-XmlReport : You cannot call a method on a null-valued expression.
At C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1:17627 char:13
+             Export-XmlReport -Result $Result -Path $Path -Format $For ...
+             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [Export-XmlReport], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull,Export-XmlReport

at Write-NUnit3OutputElement, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 17490
at Write-NUnit3TestCaseElement, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 17438
at Write-NUnit3TestSuiteElement, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 17142
at Write-NUnit3TestSuiteElement, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 17099
at Write-NUnit3TestRunChildNode, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 17000
at Write-NUnit3Report, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 16953
at Export-XmlReport, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 17779
at Export-PesterResult, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 17627
at <ScriptBlock>, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 18120
at Invoke-PluginStep, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 1788
at Invoke-Pester<End>, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 5077
at <ScriptBlock>, C:\temp\Pester_Nunit3_Bug\RunPester.ps1: line 10
at <ScriptBlock>, <No file>: line 1

at Assert-Success, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 1845
at Invoke-PluginStep, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 1807
at Invoke-Pester<End>, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 5077
at <ScriptBlock>, C:\temp\Pester_Nunit3_Bug\RunPester.ps1: line 10
at <ScriptBlock>, <No file>: line 1

or another example

System.Management.Automation.RuntimeException: Invoking step End failed:
Result 1 - Error 1:Export-XmlReport : Exception calling "ToString" with "0" argument(s): "An event was unable to invoke any of the subscribers (Exception from HRESULT: 0x80040201)"
At C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1:17627 char:13
+             Export-XmlReport -Result $Result -Path $Path -Format $For ...
+             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [Export-XmlReport], MethodInvocationException
    + FullyQualifiedErrorId : COMException,Export-XmlReport

at Write-NUnit3OutputElement, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 17490
at Write-NUnit3TestCaseElement, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 17438
at Write-NUnit3TestSuiteElement, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 17142
at Write-NUnit3TestSuiteElement, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 17099
at Write-NUnit3TestSuiteElement, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 17099
at Write-NUnit3TestRunChildNode, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 17000
at Write-NUnit3Report, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 16953
at Export-XmlReport, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 17779
at Export-PesterResult, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 17627
at <ScriptBlock>, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 18120
at Invoke-PluginStep, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 1788
at Invoke-Pester<End>, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 5077
at <ScriptBlock>, C:\projects\OneIM\v90\TST\Tests\End2End\E2E.DataImporter.Posh\RunPester.ps1: line 26
at <ScriptBlock>, <No file>: line 1

at Assert-Success, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 1845
at Invoke-PluginStep, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 1807
at Invoke-Pester<End>, C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1: line 5077
at <ScriptBlock>, C:\projects\OneIM\v90\TST\Tests\End2End\E2E.DataImporter.Posh\RunPester.ps1: line 26
at <ScriptBlock>, <No file>: line 1

Expected Behavior

Complete test result file (TestResult.OutputPath) is written. If there might be errors during processing the output-element might be skipped.

Steps To Reproduce

$sb = {
    Describe 'bug' {

        Context 'Success' {
            It 'Can bar' {
                'Hello World 1'
                'Hello World 2'
                1 | Should -Be 1
            }
        }

        Context 'Failure' {
            It 'Can foo' {
                'Hello World 1'
                $null
                'Hello World 2'
                1 | Should -Be 1
            }
        }

        Context 'No more results written into output file after failure' {
            It 'Can baz' {
                'Hello World 1'
                'Hello World 2'
                1 | Should -Be 1
            }
        }

    }
}

$container = New-PesterContainer -ScriptBlock $sb

$config = New-PesterConfiguration
$config.CodeCoverage.Enabled = $false
$config.TestResult.Enabled = $true
$config.TestResult.OutputFormat = 'NUnit3'
$config.TestResult.OutputPath = 'TestResults.xml'
$config.TestResult.TestSuiteName = 'FooBar'
$config.Output.Verbosity = 'Detailed'
$config.Run.Container = $container

Invoke-Pester -Configuration $config

Describe your environment

Pester version : 5.5.0 C:\Program Files\WindowsPowerShell\Modules\Pester\5.5.0\Pester.psm1 PowerShell version : 5.1.22621.2506 OS version : Microsoft Windows NT 10.0.22631.0

Possible Solution?

Only a suggestion. Maybe check for not null in combination with try / silent catch like

function Write-NUnit3OutputElement ($Output, [System.Xml.XmlWriter] $XmlWriter) {
    $outputString = @(foreach ($o in $Output) { 
    if ($o) {
        try {
            $o.ToString()
        }
        catch {
        }
    }
    }) -join [System.Environment]::NewLine

    $XmlWriter.WriteStartElement('output')
    $XmlWriter.WriteCData($outputString)
    $XmlWriter.WriteEndElement()
}

pester / Pester