Kernel: Cells in TreeTable rows without item cannot be edited by default

For rows with empty item (such as headlines) the editability of the
whole column was used so far.
Now such cells can only be edited if they at least contain an empty
String.

Issue-Ref: 4133
Issue-Url: #4133

Signed-off-by: Andreas Bayha <bayha@fortiss.org>

org.fortiss.tooling.common.ui/src/org/fortiss/tooling/common/ui/javafx/control/treetableview/.ratings

@@ -5,7 +5,7 @@ DynamicTextFieldTreeTableCell.java de24117e6f785b328f1ff62383626a0b4b54e8ff GREE
 DynamicTreeContentProviderBase.java 91896b1fb5104d126544c44c1ff8c30f2a13a8d6 GREEN
 DynamicTreeItem.java 7486071d20e896d6ca9a9101bf105caccf3656d0 GREEN
 DynamicTreeItemBase.java d883066ecc181120302ca32f328538de7a45b093 GREEN
-DynamicTreeTableUIProviderBase.java 7c534d8567fb88159977765cb7b6f60c21f1ab48 YELLOW
+DynamicTreeTableUIProviderBase.java 5bc7f6a74e379baa525f5d3420a95e558cbcbe90 YELLOW
 DynamicTreeTableViewer.java ead6f6671e9cb6b14632940bf440cba7e81fcd98 GREEN
 DynamicTreeUIProviderBase.java 82d3c051213f0147f4c67ad247a08696cee73110 GREEN
 DynamicTreeViewer.java 545f1ca10b7b3cad171b294a4b447875da45c9ed GREEN

org.fortiss.tooling.common.ui/src/org/fortiss/tooling/common/ui/javafx/control/treetableview/DynamicTreeTableUIProviderBase.java

@@ -48,6 +48,11 @@ import javafx.util.converter.DefaultStringConverter;
  */
 public abstract class DynamicTreeTableUIProviderBase<T> {
 	/**
+	 * Retrieves the label to be displayed in the given position.
+	 * 
+	 * Note: An empty cell can only be editable, if an empty String is returned here instead of
+	 * 'null'.
+	 * 
 	 * @param element
 	 *            the element to be displayed in the current row
 	 * @param column
@@ -347,9 +352,20 @@ public abstract class DynamicTreeTableUIProviderBase<T> {
 					@Override
 					public void updateItem(String item, boolean empty) {
 						super.updateItem(item, empty);
+
 						styleCell(this, columnIndex);
+					}
 
-						setEditable(item != null);
+					/** {@inheritDoc} */
+					@Override
+					public void startEdit() {
+						// null items are in cells, which cannot contain a value. Otherwise they
+						// would contain an empty String.
+						if(getItem() == null) {
+							return;
+						}
+
+						super.startEdit();
 					}
 				};
 
@@ -380,7 +396,7 @@ public abstract class DynamicTreeTableUIProviderBase<T> {
 		/** {@inheritDoc} */
 		@Override
 		public void startEdit() {
-			if(!isEditable() || !getTreeTableView().isEditable() ||
+			if(getItem() == null || !isEditable() || !getTreeTableView().isEditable() ||
 					!getTableColumn().isEditable()) {
 				return;
 			}