Add null-check to AbstractTableViewer and AbstractTreeViewer

Check for null before using the result of getViewerColumn(int). The
JavaDoc explicitly says that the method may return null and other places
in the code are already checking for null.

Author: fedejeanne

bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/AbstractTableViewer.java

@@ -387,9 +387,11 @@ protected void doUpdateItem(Widget widget, Object element, boolean fullMap) {
 					}
 
 					ViewerColumn columnViewer = getViewerColumn(column);
-					ViewerCell cellToUpdate = new ViewerCell(viewerRowFromItem, column, element);
 
-					columnViewer.refresh(cellToUpdate);
+					if (columnViewer != null) {
+						ViewerCell cellToUpdate = new ViewerCell(viewerRowFromItem, column, element);
+						columnViewer.refresh(cellToUpdate);
+					}
 
 					// As it is possible for user code to run the event
 					// loop check here.

bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/AbstractTreeViewer.java

@@ -994,9 +994,11 @@ protected void doUpdateItem(final Item item, Object element) {
 			}
 
 			ViewerColumn columnViewer = getViewerColumn(column);
-			ViewerCell cellToUpdate = new ViewerCell(viewerRowFromItem, column, element);
 
-			columnViewer.refresh(cellToUpdate);
+			if (columnViewer != null) {
+				ViewerCell cellToUpdate = new ViewerCell(viewerRowFromItem, column, element);
+				columnViewer.refresh(cellToUpdate);
+			}
 
 			// As it is possible for user code to run the event
 			// loop check here.