If a plugin is redeployed and a metric which has been removed is used in alert condition sets, those alerts will become unviewable from the UI.
It's debatable what should happen in this scenario, and for that I'll be logging an HPM bug. In the meantime, this NPE should be avoided so that the metric can be corrected without needing to delete and re-create the alert definition. The stacktrace is as follows and should be fairly easy to fix:
ava.lang.NullPointerException at org.hyperic.hq.ui.action.resource.common.monitor.alerts.config.ConditionBean.importProperties(ConditionBean.java:340) at org.hyperic.hq.ui.action.resource.common.monitor.alerts.config.DefinitionForm.importConditionsEnablement(DefinitionForm.java:495) at com.hyperic.hq.ui.action.resource.common.monitor.alerts.config.DefinitionForm.importConditionsEnablement(DefinitionForm.java:142) at