Crash in relation editor when saving

[GerdP]

What steps will reproduce the problem?

1. Create new relation with at least two members
2. Open relation editor with the new relation
3. Undo creation of relation
4. In relation editor change the order of the members
5. Save relation

What is the expected result?

New relation is saved

What happens instead?

Bug report

Please provide any additional information below. Attach a screenshot if possible.

Happens also with latest version

URL:https://josm.openstreetmap.de/svn/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2020-10-03 13:42:38 +0200 (Sat, 03 Oct 2020)
Build-Date:2020-10-04 01:30:47
Revision:17084
Relative:URL: ^/trunk

Identification: JOSM/1.5 (17084 en) Windows 10 64-Bit
OS Build number: Windows 10 Home 2004 (19041)
Memory Usage: 1193 MB / 3641 MB (389 MB allocated, but free)
Java version: 1.8.0_221-b11, Oracle Corporation, Java HotSpot(TM) 64-Bit Server VM
Look and Feel: com.sun.java.swing.plaf.windows.WindowsLookAndFeel
Screen: \Display0 1920x1080 (scaling 1.0x1.0)
Maximum Screen Size: 1920x1080
Best cursor sizes: 16x16 -> 32x32, 32x32 -> 32x32
VM arguments: [-XX:StartFlightRecording=name=MyRecording2,settings=d:\dbg\gerd.jfc, -XX:FlightRecorderOptions=defaultrecording=true,dumponexit=true,dumponexitpath=e:\ld\perf_20201011_204119.jfr]
Dataset consistency test: No problems found

Plugins:
+ OpeningHoursEditor (35414)
+ PolygonCutOut (v0.7)
+ apache-commons (35524)
+ buildings_tools (35574)
+ continuosDownload (91)
+ ejml (35313)
+ geotools (35169)
+ jaxb (35092)
+ jts (35122)
+ measurement (35405)
+ merge-overlap (35248)
+ o5m (35248)
+ opendata (35513)
+ pbf (35446)
+ poly (35248)
+ reverter (35556)
+ undelete (35521)
+ utilsplugin2 (35487)

Map paint styles:
- https://raw.githubusercontent.com/yopaseopor/traffic_signs_style_JOSM/master/Styles_Traffic_signs_AME.zip
- https://josm.openstreetmap.de/josmfile?page=Styles/Lit&zip=1

Last errors/warnings:
- 00112.892 E: Handled by bug report queue: java.lang.IllegalArgumentException: Parameter 'data' must not be null


=== REPORTED CRASH DATA ===
BugReportExceptionHandler#handleException:
No data collected.

Warning issued by: BugReportExceptionHandler#handleException

=== STACK TRACE ===
Thread: AWT-EventQueue-0 (20) of main
java.lang.IllegalArgumentException: Parameter 'data' must not be null
	at org.openstreetmap.josm.tools.CheckParameterUtil.ensureParameterNotNull(CheckParameterUtil.java:26)
	at org.openstreetmap.josm.command.Command.<init>(Command.java:138)
	at org.openstreetmap.josm.command.ChangeCommand.<init>(ChangeCommand.java:49)
	at org.openstreetmap.josm.command.ChangeCommand.<init>(ChangeCommand.java:37)
	at org.openstreetmap.josm.gui.dialogs.relation.actions.SavingAction.applyExistingNonConflictingRelation(SavingAction.java:89)
	at org.openstreetmap.josm.gui.dialogs.relation.actions.SavingAction.applyChanges(SavingAction.java:162)
	at org.openstreetmap.josm.gui.dialogs.relation.actions.OKAction.actionPerformed(OKAction.java:33)
	at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
	at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
	at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
	at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
	at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
	at java.awt.AWTEventMulticaster.mouseReleased(Unknown Source)
	at java.awt.Component.processMouseEvent(Unknown Source)
	at javax.swing.JComponent.processMouseEvent(Unknown Source)
	at java.awt.Component.processEvent(Unknown Source)
	at java.awt.Container.processEvent(Unknown Source)
	at java.awt.Component.dispatchEventImpl(Unknown Source)
	at java.awt.Container.dispatchEventImpl(Unknown Source)
	at java.awt.Component.dispatchEvent(Unknown Source)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
	at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
	at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
	at java.awt.Container.dispatchEventImpl(Unknown Source)
	at java.awt.Window.dispatchEventImpl(Unknown Source)
	at java.awt.Component.dispatchEvent(Unknown Source)
	at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
	at java.awt.EventQueue.access$500(Unknown Source)
	at java.awt.EventQueue$3.run(Unknown Source)
	at java.awt.EventQueue$3.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
	at java.awt.EventQueue$4.run(Unknown Source)
	at java.awt.EventQueue$4.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
	at java.awt.EventQueue.dispatchEvent(Unknown Source)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
	at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
	at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
	at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
	at java.awt.EventDispatchThread.run(Unknown Source)