CA-271945: Attempting to resolve the pool's connection and searching for it in all open connections for the EnableHAAction in case a host has been destroyed or the master changed.

Preventing an Object (host) has been deleted error when attempting to reenable Maintenance Mode on a Host that has been kicked out of the pool when exiting the RPU Wizard.
Returning null to prevent making orphened tasks that will be stuck at 0% completion if it is impossible to fulfill.

Signed-off-by: Aaron Robson <aaron.robson@citrix.com>

XenAdmin/Diagnostics/Problems/HostProblem/HostMaintenanceMode.cs

@@ -63,8 +63,12 @@ namespace XenAdmin.Diagnostics.Problems.HostProblem
 
         public override AsyncAction CreateUnwindChangesAction()
         {
-            Program.MainWindow.CloseActiveWizards(Server.Connection);
+            var server = Server.Connection.Resolve(new XenRef<Host>(Server.opaque_ref));
-            return new DisableHostAction(Server);
+            if (server == null)
+                return null;
+
+            Program.MainWindow.CloseActiveWizards(server.Connection);
+            return new DisableHostAction(server);
         }
 
     }

XenAdmin/Diagnostics/Problems/PoolProblem/HAEnabledProblem.cs

@@ -58,7 +58,18 @@ namespace XenAdmin.Diagnostics.Problems.PoolProblem
 
         public override AsyncAction CreateUnwindChangesAction()
         {
-            return new EnableHAAction(Pool, null, HeartbeatSrs, FailuresToTolerate);
+            var pool = Pool.Connection.Resolve(new XenRef<Pool>(Pool.opaque_ref));
+            if (pool == null)
+            {
+                foreach (var xenConnection in ConnectionsManager.XenConnectionsCopy)
+                {
+                    pool = xenConnection.Resolve(new XenRef<Pool>(Pool.opaque_ref));
+                    if (pool != null)
+                        break;
+                }
+            }
+
+            return pool != null ? new EnableHAAction(pool, null, HeartbeatSrs, FailuresToTolerate) : null;
         }
 
         public override string Description

XenAdmin/Diagnostics/Problems/VMProblem/AutoStartEnabled.cs

@@ -88,6 +88,9 @@ namespace XenAdmin.Diagnostics.Problems.VMProblem
 
         public override AsyncAction CreateUnwindChangesAction()
         {
+            if (VM.Connection.Resolve(new XenRef<VM>(VM.opaque_ref)) == null) // check if the vm is still in the cache
+                return null;
+
             return new DelegatedAsyncAction(
                VM.Connection,
                Messages.ACTION_ENABLE_AUTOSTART_ON_VM,