Add validation for dump directory existence and null checks

Co-authored-by: Evangelink <11340282+Evangelink@users.noreply.github.com>

Author: Copilot

src/Platform/Microsoft.Testing.Extensions.CrashDump/CrashDumpProcessLifetimeHandler.cs

@@ -66,7 +66,12 @@ public async Task OnTestHostProcessExitedAsync(ITestHostProcessInformation testH
         await _outputDisplay.DisplayAsync(this, new ErrorMessageOutputDeviceData(string.Format(CultureInfo.InvariantCulture, CrashDumpResources.CrashDumpProcessCrashedDumpFileCreated, testHostProcessInformation.PID)), cancellation).ConfigureAwait(false);
 
         string expectedDumpFile = _netCoreCrashDumpGeneratorConfiguration.DumpFileNamePattern.Replace("%p", testHostProcessInformation.PID.ToString(CultureInfo.InvariantCulture));
-        string dumpDirectory = Path.GetDirectoryName(expectedDumpFile)!;
+        string? dumpDirectory = Path.GetDirectoryName(expectedDumpFile);
+        if (string.IsNullOrEmpty(dumpDirectory) || !Directory.Exists(dumpDirectory))
+        {
+            await _outputDisplay.DisplayAsync(this, new ErrorMessageOutputDeviceData(string.Format(CultureInfo.InvariantCulture, CrashDumpResources.CannotFindExpectedCrashDumpFile, expectedDumpFile)), cancellation).ConfigureAwait(false);
+            return;
+        }
 
         // Collect all dump files in the directory to capture crashes from child processes
         bool foundExpectedDump = false;