rem: if (gDBView.sortType == nsMsgViewSortType.byThread) { 
add: if (gDBView && (gDBView.sortType == nsMsgViewSortType.byThread)) { 
context: function ShowAppropriateColumns(){    if (gDBView.sortType == nsMsgViewSortType.byThread) {      // don't hide them when sorted by thread      SetHiddenAttributeOnThreadOnlyColumns("");    }    else {      // hide them when not sorted by thread      SetHiddenAttributeOnThreadOnlyColumns("true");    }}
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