Fix NullReferenceException when calling Lock.Acquire with CancellationToken

Consul.Test/LockTest.cs

@@ -38,7 +38,7 @@ public void Dispose()
         {
             m_lock.Dispose();
         }
-    
+
         [Fact]
         public async Task Lock_AcquireRelease()
         {
@@ -669,5 +669,26 @@ public async Task Lock_DeleteKey()
                 }
             }
         }
+
+        [Fact]
+        public async Task Lock_AcquireTimeout()
+        {
+            using (var client = new ConsulClient())
+            {
+                const string keyName = "test/lock/acquiretimeout";
+                var distributedLock = await client.AcquireLock(keyName);
+                try
+                {
+                    using (var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5)))
+                    {
+                        await Assert.ThrowsAsync<LockNotHeldException>(() => client.AcquireLock(keyName, cts.Token));
+                    }
+                }
+                finally
+                {
+                    await distributedLock.Release();
+                }
+            }
+        }
     }
 }

Consul/Lock.cs

@@ -331,11 +331,21 @@ public async Task<CancellationToken> Acquire(CancellationToken ct)
                 if (ct.IsCancellationRequested || (!IsHeld && !string.IsNullOrEmpty(Opts.Session)))
                 {
                     DisposeCancellationTokenSource();
-                    if (_sessionRenewTask != null)
+                    if (_monitorTask != null)
                     {
                         try
                         {
                             await _monitorTask.ConfigureAwait(false);
+                        }
+                        catch (AggregateException)
+                        {
+                            // Ignore AggregateExceptions from the tasks during Release, since if the Monitor task died, the developer will be Super Confused if they see the exception during Release.
+                        }
+                    }
+                    if (_sessionRenewTask != null)
+                    {
+                        try
+                        {
                             await _sessionRenewTask.ConfigureAwait(false);
                         }
                         catch (AggregateException)