Not possible to open the multibranch timeline if some element aren't available and visible in the workspace

When launching the concurrent engine (Sigpml or tfsm)

the timeline may fail with
```

java.lang.NullPointerException
at org.eclipse.gemoc.timeline.view.AbstractTimelineView.setTimelineProvider(AbstractTimelineView.java:638)
at org.eclipse.gemoc.addon.eventscheduling.timeline.views.timeline.EventSchedulingTimeLineView.disposeTimeLineProvider(EventSchedulingTimeLineView.java:188)
at org.eclipse.gemoc.addon.eventscheduling.timeline.views.timeline.EventSchedulingTimeLineView.configure(EventSchedulingTimeLineView.java:142)
at org.eclipse.gemoc.addon.eventscheduling.timeline.views.timeline.EventSchedulingTimeLineView.update(EventSchedulingTimeLineView.java:293)
at org.eclipse.gemoc.addon.eventscheduling.timeline.views.timeline.EventSchedulingTimeLineView.engineSelectionChanged(EventSchedulingTimeLineView.java:194)
at org.eclipse.gemoc.executionframework.ui.views.engine.EnginesStatusView.fireEngineSelectionChanged(EnginesStatusView.java:343)
at org.eclipse.gemoc.executionframework.ui.views.engine.EnginesStatusView$1.selectionChanged(EnginesStatusView.java:89)
at org.eclipse.jface.viewers.Viewer$1.run(Viewer.java:151)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:45)
```

or
```
java.lang.NullPointerException
at org.eclipse.gemoc.timeline.view.AbstractTimelineView.setTimelineProvider(AbstractTimelineView.java:638)
at org.eclipse.gemoc.addon.eventscheduling.timeline.views.timeline.EventSchedulingTimeLineView.configure(EventSchedulingTimeLineView.java:151)
at org.eclipse.gemoc.addon.eventscheduling.timeline.views.timeline.EventSchedulingTimeLineView.init(EventSchedulingTimeLineView.java:84)
at org.eclipse.ui.part.ViewPart.init(ViewPart.java:113)
at org.eclipse.ui.internal.ViewReference.initialize(ViewReference.java:129)
```
Actually, once it crashed that way, it not possible to open it without restarting the eclipse

the only way I found to correctly use the timeline is to:
- first open the timeline and make sure is it visible (if it is stacked in a tab and not visible, the same error occurs) (the exception varies depending if it is not present or not visible)
- then launch the simulation (with the multibranch addon)