Commented Unassigned: NullReferenceException when display Run Times in TaskEditDialog [12302]

When I try displaying the schedule for the an __existing__ task, I see the following error in the Run Times tab.

Unable to show the run times for this task.
Error: System.NullReferenceException: Object references not set to an instance of an object.
at Microsoft.Win32.TaskScheduler.TaskService.NewTask()
at Microsoft.Win32.TaskScheduler.TaskPropertiesControl.runTimesTab_Enter(Object sender, EventArgs e)

If I click ok, Windows display a Quit, Continue, Ignore dialog box and nothing gets saved.

Note that is only happening when I try to modify an existing task. If I create a new task, the RunTimes tab gets filled correctly.
Comments: ** Comment from web user: petarian **

In addition to my last message, I get the following stack trace when I click OK in the dialog box.

System.NullReferenceException: Object reference not set to an instance of an object.
at Microsoft.Win32.TaskScheduler.TaskFolder.RegisterTaskDefinition(String Path, TaskDefinition definition, TaskCreation createType, String UserId, String password, TaskLogonType LogonType, String sddl)
at Microsoft.Win32.TaskScheduler.TaskFolder.RegisterTaskDefinition(String Path, TaskDefinition definition)
at Microsoft.Win32.TaskScheduler.Task.RegisterChanges()
at Microsoft.Win32.TaskScheduler.TaskEditDialog.okBtn_Click(Object sender, EventArgs e)
at System.Windows.Forms.Control.OnClick(EventArgs e)
at System.Windows.Forms.Button.OnClick(EventArgs e)