Fixed a null reference exception in the welcome screen tile definition

[Mailaender]

chemclipse/chemclipse/plugins/org.eclipse.chemclipse.ux.extension.ui/src/org/eclipse/chemclipse/ux/extension/ui/definitions/TileDefinition.java

Lines 85 to 93 in abcab84

	/**
	* A tile can proclaim that it wants to be executed in a a given perspective, if the perspective is really switched might depend on the implementor
	*
	* @return the preferred perspective for this tile or <code>null</code> if there is no preference
	*/
	default String getPreferredPerspective() {
	return null;
	}

promises this can be left as the default null, but that would crash.