SL-15387 Resolve some columns crashes

Looks like mColumnsIndexed had dead pointers which resulted in a crash and there is some kind of hard to trigger interaction with searchable UI

indra/llui/llscrolllistctrl.cpp

@@ -336,8 +336,7 @@ LLScrollListCtrl::~LLScrollListCtrl()
 
 	std::for_each(mItemList.begin(), mItemList.end(), DeletePointer());
 	mItemList.clear();
-	std::for_each(mColumns.begin(), mColumns.end(), DeletePairedPointer());
-	mColumns.clear();
+    clearColumns(); //clears columns and deletes headers
 	delete mIsFriendSignal;
 }
 
@@ -3011,6 +3010,8 @@ void LLScrollListCtrl::clearColumns()
 	mSortColumns.clear();
 	mTotalStaticColumnWidth = 0;
 	mTotalColumnPadding = 0;
+
+    dirtyColumns(); // Clears mColumnsIndexed
 }
 
 void LLScrollListCtrl::setColumnLabel(const std::string& column, const std::string& label)

indra/newview/llfloaterpreference.cpp

@@ -2958,6 +2958,9 @@ void LLPanelPreferenceControls::populateControlTable()
         LL_WARNS() << "Unimplemented mode" << LL_ENDL;
     }
 
+    // explicit update to make sure table is ready for llsearchableui
+    pControlsTable->updateColumns();
+
     // Searchable columns were removed and readded, mark searchables for an update
     // Note: at the moment tables/lists lack proper llsearchableui support
     LLFloaterPreference* instance = LLFloaterReg::findTypedInstance<LLFloaterPreference>("preferences");