Layers widget requires selected content

[alansemenov]

Layers widget produces an error when no content is selected.

It should correctly handle this and give "No content selected" message inside the panel, like other widgets.

Caused by: com.enonic.xp.resource.ResourceProblemException: No method or field with name 'contentId' on line 3
	at com.enonic.xp.resource.ResourceProblemException$Builder.build(ResourceProblemException.java:131)
	at com.enonic.lib.mustache.MustacheProcessor.handleError(MustacheProcessor.java:101)
	at com.enonic.lib.mustache.MustacheProcessor.handleError(MustacheProcessor.java:88)
	at com.enonic.lib.mustache.MustacheProcessor.execute(MustacheProcessor.java:66)
	at com.enonic.lib.mustache.MustacheProcessor.process(MustacheProcessor.java:48)
	at jdk.scripting.nashorn.scripts/jdk.nashorn.internal.scripts.Script$Recompilation$6762$558AA$mustache.L:1#render(com.enonic.app.contentstudio.plus:/lib/mustache.js:25)
	at jdk.scripting.nashorn.scripts/jdk.nashorn.internal.scripts.Script$Recompilation$6758$250A$layers.L:1#renderWidgetView(com.enonic.app.contentstudio.plus:/admin/widgets/layers/layers.js:27)
	at jdk.scripting.nashorn.scripts/jdk.nashorn.internal.scripts.Script$Recompilation$6757$187A$layers.L:1#handleGet(com.enonic.app.contentstudio.plus:/admin/widgets/layers/layers.js:6)