Complete rendering for Signature Attribute

[eparovyshnaya]

Signature attribute does not supply representative icon, and thus can fail tree rendering

java.lang.NullPointerException: Cannot invoke "org.eclipse.swt.graphics.Image.getImageData()" because "image" is null
at org.eclipse.emf.edit.ui.provider.ComposedImageDescriptor.getSize(ExtendedImageRegistry.java:407)
at org.eclipse.jface.resource.CompositeImageDescriptor.getImageData(CompositeImageDescriptor.java:367)
at org.eclipse.swt.internal.DPIUtil.validateAndGetImageDataAtZoom(DPIUtil.java:408)
at org.eclipse.swt.graphics.Image.(Image.java:717)
at org.eclipse.jface.resource.ImageDescriptor.createImage(ImageDescriptor.java:341)
at org.eclipse.jface.resource.ImageDescriptor.createImage(ImageDescriptor.java:290)
at org.eclipse.jface.resource.ImageDescriptor.createImage(ImageDescriptor.java:268)
at org.eclipse.emf.edit.ui.provider.ExtendedImageRegistry.getImage(ExtendedImageRegistry.java:185)
at org.eclipse.emf.edit.ui.provider.ExtendedImageRegistry.getImageDescriptor(ExtendedImageRegistry.java:205)
at org.eclipse.emf.edit.ui.action.StaticSelectionCommandAction.objectToImageDescriptor(StaticSelectionCommandAction.java:245)
at org.eclipse.emf.edit.ui.action.StaticSelectionCommandAction.configureAction(StaticSelectionCommandAction.java:173)
at org.eclipse.emf.edit.ui.action.CreateChildAction.(CreateChildAction.java:47)
at org.eclipse.emf.edit.ui.action.CreateChildAction.(CreateChildAction.java:57)