Eclipse E4 Model Fragment LazyStackRenderer NullPointerException on Restart
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I created one plugin with the "Eclipse E4 Application Project" wizard, then added a second  plugin with a model Fragment which contributes a Part to the one PartStack of the E4 application.

For this I followed http://www.vogella.com/articles/Eclipse4Modularity/article.html
: Set ID of PartStack in the original Application.e4xmi, then define fragment.xmi that adds a Part to the PartStack.

All works fine when executed for the *first time*, or executed with a *new workspace* (by adding "-clearPersistedState" to the program arguments).
But when starting the application with an existing workspace that contains the persisted workbench.xmi, there is this error message:

    !MESSAGE Problems occurred when invoking code from plug-in: "org.eclipse.e4.ui.workbench.swt".
    java.lang.NullPointerException
	at org.eclipse.e4.ui.workbench.renderers.swt.LazyStackRenderer.showTab(LazyStackRenderer.java:156)
	at org.eclipse.e4.ui.workbench.renderers.swt.StackRenderer.showTab(StackRenderer.java:1024)
	at org.eclipse.e4.ui.workbench.renderers.swt.LazyStackRenderer.postProcess(LazyStackRenderer.java:98)
	at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine.safeCreateGui(PartRenderingEngine.java:639)
	at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine.safeCreateGui(PartRenderingEngine.java:725)
	at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine.access$2(PartRenderingEngine.java:696)
	at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine$7.run(PartRenderingEngine.java:690)
	at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
	at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine.createGui(PartRenderingEngine.java:675)


Debugging into LazyStackRenderer.java:156 indicates that a getParent() call returns null.

The problem seems in re-loading the persisted workbench.xmi, maybe because some IDs are not properly defined, but I cannot determine the exact reason.

This is for Eclipse 4.2 on Windows 7.