Here's another issue I didn't see doc'ed anywhere. Hopefully this will help you out if you're seeing the OpsMgr console crash with a NullReferenceException.
Symptoms: When changing an alert property in an alerts view with alerts displayed from multiple connected management group tiers (Show Connected Alerts option enabled), the Operations Manager console may crash with a NullReferenceException.
Cause: If one or more tiers in a connected management group scenario have different alert resolution states configured and the steps in the "Steps to Reproduce" section are followed, attempting to change the properties of an affected alert (for example, changing the owner, Ticket ID, or amending Custom Fields), the UI will crash.
Repro Steps: This issue can be seen if the Alert Resolution States for multiple management groups tiers are different (For example in Tier 1 a resolution state Assigned to Exchange Admin = 30, where this does not exist in Tier 2). Using this example:
Resolution: To correct the situation restart the console and change the alerts with a numeric resolution state to a correct resolution state for that tier by selecting the alerts for that tier and selecting Set Resolution State from the right mouse click context menu.
NOTE: When multi-selecting alerts for different tiers the mouse right click context menu Set Resolution State is not available. By selecting Properties instead it possible to select an Alert resolution state which causes this problem as the combo box is not disabled in the same way.
More Information: This situation can be prevented in several ways:
If possible, utilize the same alert resolution states and ID's within each connected management group.
If this is not possible, make console operators aware of the issue and do not use the Alert Status combo box when selecting multiple alerts for connected management groups
NOTE: This applies to System Center Operations Manager 2007 & System Center Operations Manager 2007 Service Pack 1 (SP1).
Hope this helps,
J.C. Hornbeck | Manageability Knowledge Engineer