fix processex.cs possible problems (#22347)

- `_output ` May be executed in synchronous and asynchronous situations
- Process Is a nullable

src/Shared/Process/ProcessEx.cs

@@ -196,8 +196,12 @@ namespace Microsoft.AspNetCore.Internal
 
             var exited = Exited.Wait(timeSpan.Value);
             if (!exited)
+            {
+                lock (_testOutputLock)
                 {
                     _output.WriteLine($"The process didn't exit within the allotted time ({timeSpan.Value.TotalSeconds} seconds).");
+                }
+
                 _process.Dispose();
             }
             else if (assertSuccess && _process.ExitCode != 0)
@@ -227,6 +231,8 @@ namespace Microsoft.AspNetCore.Internal
                 _process.KillTree();
             }
 
+            if (_process != null)
+            {
                 _process.CancelOutputRead();
                 _process.CancelErrorRead();
 
@@ -236,4 +242,5 @@ namespace Microsoft.AspNetCore.Internal
                 _process.Dispose();
             }
         }
+    }
 }