Bug 265856 [Viewers] TreeViewer Widget Dispose Error
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/SWTFocusCellManager.java
if( event.selection.isEmpty() ) {
					setFocusCell(null);
				}
			}
		});
		viewer.getControl().addListener(SWT.FocusIn, listener);
	}
	/**
	 * @return the cell with the focus
	 *
	 */
	public ViewerCell getFocusCell() {
		return focusCell;
	}
	void setFocusCell(ViewerCell focusCell) {
		ViewerCell oldCell = this.focusCell;
		if( this.focusCell != null && ! this.focusCell.getItem().isDisposed() ) {
			this.focusCell.getItem().removeDisposeListener(itemDeletionListener);
		}
		this.focusCell = focusCell;
		if( this.focusCell != null && ! this.focusCell.getItem().isDisposed() ) {
			this.focusCell.getItem().addDisposeListener(itemDeletionListener);
		}
		this.cellHighlighter.focusCellChanged(focusCell,oldCell);
	}
	ColumnViewer getViewer() {
		return viewer;
	}
}