Bug 260892 [CellEditors] NPE from ColumnViewerEditor caused by showing a dialog during modify
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/ColumnViewerEditor.java
if( ! this.cell.getItem().isDisposed() ) {
						this.cell.getItem().addDisposeListener(disposeListener);
					}
					
					this.cellEditor = null;
					this.cell = null;
				}
			} finally {
				inEditorDeactivation = false;
			}
		}
	}
	/**
	 * Enable the editor by mouse down
	 *
	 * @param event
	 */
	void handleEditorActivationEvent(ColumnViewerEditorActivationEvent event) {
		
		if (!event.cancel
				&& editorActivationStrategy.isEditorActivationEvent(event)) {
			if (cellEditor != null) {
				applyEditorValue();
			}
			this.cell = (ViewerCell) event.getSource();
			if( ! activateCellEditor(event) ) {
				this.cell = null;
				this.cellEditor = null;
			}