Bug 51110 [Preferences] Error notifying a preference listener on font change.
M6 Create a font definition with no defaultsTo attribute. Go in to preferences and change the value of Text Font and hit OK you will get an error because you try call JFaceResources.getFontRegistry().put() with a null value.
bundles/org.eclipse.ui/src/org/eclipse/ui/internal/UIPlugin.java
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 * Copyright (c) 2000, 2003 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials 
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 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/cpl-v10.html
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 * Contributors:
 *     IBM Corporation - initial API and implementation
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package org.eclipse.ui.internal;
import org.eclipse.core.runtime.IPluginDescriptor;
import org.eclipse.jface.preference.IPreferenceStore;
import org.eclipse.jface.resource.ImageRegistry;
import org.eclipse.jface.util.Assert;
import org.eclipse.ui.IWorkbenchPreferenceConstants;
import org.eclipse.ui.plugin.AbstractUIPlugin;