NPE: using editor.collapsible surrounding digram editors nested within digram.box crashes the IDE

Setup:

> Diagram Editor
> > Diagram.Box editor
>>> some editor cells
>>> collapsible cell
>>>> diagram editor

the crash is triggered by collapsing and expanding the collapsible

Stack Trace:

```
java.lang.NullPointerException
at de.itemis.mps.editor.diagram.runtime.jgraph.MPSCell.getExpectedX(MPSCell.java:90)
at de.itemis.mps.editor.diagram.runtime.jgraph.MPSCell.moveCell(MPSCell.java:132)
at de.itemis.mps.editor.diagram.runtime.jgraph.MPSCell.setStateRelativePos(MPSCell.java:102)
at de.itemis.mps.editor.diagram.runtime.jgraph.BoxBaseDCell.updateMPSCellPositions(BoxBaseDCell.java:102)
at de.itemis.mps.editor.diagram.runtime.jgraph.MyGraphView$1.lambda$layout$0(MyGraphView.java:59)
at de.itemis.mps.editor.diagram.runtime.ContextVariables.lambda$withValue$0(ContextVariables.java:39)
at de.itemis.mps.editor.diagram.runtime.ContextVariables.enterNewContext(ContextVariables.java:30)
at de.itemis.mps.editor.diagram.runtime.ContextVariables.withValue(ContextVariables.java:37)
at de.itemis.mps.editor.diagram.runtime.jgraph.BaseDiagramECell.withContextGraph(BaseDiagramECell.java:120)
at de.itemis.mps.editor.diagram.runtime.jgraph.MyGraphView$1.lambda$layout$1(MyGraphView.java:57)
at de.itemis.mps.editor.diagram.runtime.model.ScalableEditorCell.withScalingDisabled(ScalableEditorCell.java:27)
at de.itemis.mps.editor.diagram.runtime.jgraph.MyGraphView$1.layout(MyGraphView.java:57)
at de.itemis.mps.editor.diagram.runtime.jgraph.LayoutDependencies.layoutAndRecordDependencies(LayoutDependencies.java:81)
at de.itemis.mps.editor.diagram.runtime.jgraph.MyGraphView.updateCellState(MyGraphView.java:49)
at com.mxgraph.view.mxGraphView.validateCellState(mxGraphView.java:597)
at com.mxgraph.view.mxGraphView.validateCellState(mxGraphView.java:559)
at com.mxgraph.view.mxGraphView.validateCellState(mxGraphView.java:612)
at com.mxgraph.view.mxGraphView.validateCellState(mxGraphView.java:559)
at com.mxgraph.view.mxGraphView.validateCellState(mxGraphView.java:612)
at com.mxgraph.view.mxGraphView.validateCellState(mxGraphView.java:559)
at com.mxgraph.view.mxGraphView.validateCellState(mxGraphView.java:612)
at com.mxgraph.view.mxGraphView.validateCellState(mxGraphView.java:559)
at com.mxgraph.view.mxGraphView.validateCellState(mxGraphView.java:612)
at com.mxgraph.view.mxGraphView.validateCellState(mxGraphView.java:559)
at com.mxgraph.view.mxGraphView.validate(mxGraphView.java:450)
at de.itemis.mps.editor.diagram.runtime.jgraph.RootDiagramECell$5.lambda$layout$0(RootDiagramECell.java:430)
at de.itemis.mps.editor.diagram.runtime.ContextVariables.lambda$withValue$0(ContextVariables.java:39)
at de.itemis.mps.editor.diagram.runtime.ContextVariables.enterNewContext(ContextVariables.java:30)
at de.itemis.mps.editor.diagram.runtime.ContextVariables.withValue(ContextVariables.java:37)
at de.itemis.mps.editor.diagram.runtime.jgraph.BaseDiagramECell.withContextGraph(BaseDiagramECell.java:120)
at de.itemis.mps.editor.diagram.runtime.jgraph.RootDiagramECell$5.lambda$layout$1(RootDiagramECell.java:407)
at de.itemis.mps.editor.diagram.runtime.model.ScalableEditorCell.withScalingDisabled(ScalableEditorCell.java:27)
at de.itemis.mps.editor.diagram.runtime.jgraph.RootDiagramECell$5.lambda$layout$2(RootDiagramECell.java:407)
at de.itemis.mps.editor.diagram.runtime.jgraph.JGraphModelSynchronizer.suspend(JGraphModelSynchronizer.java:172)
at de.itemis.mps.editor.diagram.runtime.jgraph.RootDiagramECell$5.layout(RootDiagramECell.java:407)
at de.itemis.mps.editor.diagram.runtime.jgraph.LayoutDependencies.layoutAndRecordDependencies(LayoutDependencies.java:81)
at de.itemis.mps.editor.diagram.runtime.jgraph.RootDiagramECell.relayoutImpl(RootDiagramECell.java:404)
at jetbrains.mps.nodeEditor.cells.EditorCell_Basic.relayout(EditorCell_Basic.java:789)
at jetbrains.mps.nodeEditor.EditorComponent.doRelayout(EditorComponent.java:1857)
at jetbrains.mps.nodeEditor.EditorComponent.relayout(EditorComponent.java:1850)
at de.itemis.mps.editor.collapsible.runtime.CollapsibleCell.setCollapsibleCollapsed(CollapsibleCell.java:248)
at de.itemis.mps.editor.diagram.runtime.jgraph.DiagramCollapsibleCell.setCollapsibleCollapsed(DiagramCollapsibleCell.java:36)
at de.itemis.mps.editor.collapsible.runtime.CollapseExpandToggleCell.toggle(CollapseExpandToggleCell.java:56)
at de.itemis.mps.editor.diagram.runtime.jgraph.MyGraphHandler.mousePressed(MyGraphHandler.java:87)
at java.desktop/java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:288)
at java.desktop/java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:287)
at java.desktop/java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:287)
at java.desktop/java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:287)
at java.desktop/java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:287)
at java.desktop/java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:287)
at java.desktop/java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:287)
at java.desktop/java.awt.Component.processMouseEvent(Component.java:6651)
at java.desktop/javax.swing.JComponent.processMouseEvent(JComponent.java:3345)
at de.itemis.mps.editor.diagram.runtime.jgraph.MyGraphComponent$MyGraphControl.processMouseEvent(MyGraphComponent.java:560)
at java.desktop/java.awt.Component.processEvent(Component.java:6419)
at java.desktop/java.awt.Container.processEvent(Container.java:2263)
at java.desktop/java.awt.Component.dispatchEventImpl(Component.java:5029)
at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2321)
at java.desktop/java.awt.Component.dispatchEvent(Component.java:4861)
at java.desktop/java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4918)
at java.desktop/java.awt.LightweightDispatcher.processMouseEvent(Container.java:4544)
at java.desktop/java.awt.LightweightDispatcher.dispatchEvent(Container.java:4488)
at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2307)
at java.desktop/java.awt.Window.dispatchEventImpl(Window.java:2790)
at java.desktop/java.awt.Component.dispatchEvent(Component.java:4861)
at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:778)
at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:727)
at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721)
at java.base/java.security.AccessController.doPrivileged(Native Method)
at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:95)
at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:751)
at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:749)
at java.base/java.security.AccessController.doPrivileged(Native Method)
at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:748)
at com.intellij.ide.IdeEventQueue.defaultDispatchEvent(IdeEventQueue.java:891)
at com.intellij.ide.IdeEventQueue.dispatchMouseEvent(IdeEventQueue.java:820)
at com.intellij.ide.IdeEventQueue._dispatchEvent(IdeEventQueue.java:757)
at com.intellij.ide.IdeEventQueue.lambda$dispatchEvent$6(IdeEventQueue.java:447)
at com.intellij.openapi.progress.impl.CoreProgressManager.computePrioritized(CoreProgressManager.java:818)
at com.intellij.ide.IdeEventQueue.lambda$dispatchEvent$7(IdeEventQueue.java:446)
at com.intellij.openapi.application.impl.ApplicationImpl.runIntendedWriteActionOnCurrentThread(ApplicationImpl.java:805)
at com.intellij.ide.IdeEventQueue.dispatchEvent(IdeEventQueue.java:498)
at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)
```

Start by reproducing the nested Diagram Editor, Diagram.Box, and collapsible diagram setup, then collapse and expand the cell. Trace MPSCell.getExpectedX and BoxBaseDCell.updateMPSCellPositions using the reported stack trace; done means the same operation no longer throws a NullPointerException.

Written by the indexing model from the issue text.