Bug 299685 Empty iconURI's cause 'MalformedURLException' failures rather than being ignored
One of the issues Lars' found is that if you fill in a field (i.e. iconURI) and subsequently clear it you don't end up in the same model state. Once the value has been set in the editor it will never be 'null' again, clearing the field just sets it to an empty string.
bundles/org.eclipse.e4.ui.workbench.renderers.swt/src/org/eclipse/e4/workbench/ui/renderers/swt/WBWRenderer.java
data.heightHint = 250;
			data.widthHint = 300;
			tableViewer.getControl().setLayoutData(data);
			tableViewer.setLabelProvider(new LabelProvider() {
				@Override
				public String getText(Object element) {
					return ((MSaveablePart) element).getLabel();
				}
			});
			tableViewer.setContentProvider(ArrayContentProvider.getInstance());
			tableViewer.setInput(collection);
			tableViewer.setAllChecked(true);
			return parent;
		}
		@Override
		public void create() {
			super.create();
			applyDialogStyles(getShell());
		}
		@Override
		protected void okPressed() {
			checkedElements = tableViewer.getCheckedElements();
			super.okPressed();
		}
		public Object[] getCheckedElements() {
			return checkedElements;
		}
	}
}