Fix a null reference crash in completion code (#19916)

src/System.Management.Automation/engine/CommandCompletion/CommandCompletion.cs

@@ -516,7 +516,8 @@ namespace System.Management.Automation
                     // If we were invoked from TabExpansion2, we want to "remove" TabExpansion2 and anything it calls
                     // from our results.  We do this by faking out the session so that TabExpansion2 isn't anywhere to be found.
                     SessionStateScope scopeToRestore;
-                    if (context.CurrentCommandProcessor.Command.CommandInfo.Name.Equals("TabExpansion2", StringComparison.OrdinalIgnoreCase)
+                    if (context.CurrentCommandProcessor is not null
+                        && context.CurrentCommandProcessor.Command.CommandInfo.Name.Equals("TabExpansion2", StringComparison.OrdinalIgnoreCase)
                         && context.CurrentCommandProcessor.UseLocalScope
                         && context.EngineSessionState.CurrentScope.Parent is not null)
                     {

test/powershell/Host/TabCompletion/BugFix.Tests.ps1

@@ -87,4 +87,21 @@ Describe "Tab completion bug fix" -Tags "CI" {
         $result.CompletionMatches[0].CompletionText | Should -BeExactly 'Ascending'
         $result.CompletionMatches[1].CompletionText | Should -BeExactly 'Descending'
     }
+
+    It "Issue#19912 - Tab completion should not crash" {
+        $ISS = [initialsessionstate]::CreateDefault()
+        $Runspace = [runspacefactory]::CreateRunspace($ISS)
+        $Runspace.Open()
+        $OldRunspace = [runspace]::DefaultRunspace
+        try
+        {
+            [runspace]::DefaultRunspace = $Runspace
+            {[System.Management.Automation.CommandCompletion]::CompleteInput('Get-', 3, $null)} | Should -Not -Throw
+        }
+        finally
+        {
+            [runspace]::DefaultRunspace = $OldRunspace
+            $Runspace.Dispose()
+        }
+    }
 }