carsh on deleting scoutsign layer

[malenki]

Steps to reproduce:

• Start a JOSM with Scoutsigns installed
• create a new data layer
• delete both data and scoutsigns layer with one go (or first the data and then the scoutsigns layer)

The result is the following exception:

Repository Root: http://josm.openstreetmap.de/svn
Build-Date: 2015-01-08 02:34:21
Last Changed Author: bastiK
Revision: 7935
Repository UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Relative URL: ^/trunk
URL: http://josm.openstreetmap.de/svn/trunk
Last Changed Date: 2015-01-07 21:01:44 +0100 (Wed, 07 Jan 2015)
Last Changed Rev: 7935

Identification: JOSM/1.5 (7935 de) Linux Debian GNU/Linux 8.0 (jessie)
Memory Usage: 697 MB / 12444 MB (586 MB allocated, but free)
Java version: 1.7.0_65, Oracle Corporation, OpenJDK 64-Bit Server VM
Java package: openjdk-7-jre:amd64-7u71-2.5.3-2
VM arguments: [-Dsun.java2d.opengl=false]

Plugins:
- scoutsigns (21)

Last errors/warnings:
- E: java.lang.NullPointerException

java.lang.NullPointerException
	at
org.openstreetmap.josm.plugins.scoutsigns.ScoutSignsPlugin$2.run(Unknown
Source) at
java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:312) at
java.awt.EventQueue.dispatchEventImpl(EventQueue.java:733) at
java.awt.EventQueue.access$200(EventQueue.java:103) at
java.awt.EventQueue$3.run(EventQueue.java:694) at
java.awt.EventQueue$3.run(EventQueue.java:692) at
java.security.AccessController.doPrivileged(Native Method) at
java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:703) at
java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at
java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at
java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at
java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at
java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91) }}}