"Failed to locate image 'null'" after downloading a Map Paint Style

[rickmastfan67]

Tested this with a clean JOSM preference folder.

How to reproduce:

1. start JOSM
2. open preferences -> Settings for Map/Data projection -> Map Paint Styles
3. Select any of them (excluding the two internal ones) and hit the arrow to move it to the Active section on the right
4. Hit OK in your preference window and then close JOSM.
5. Restart JOSM then and notice in the CL the following message:

   Failed to locate image 'null'
   

If you delete that Map Paint Style, and then restart, that message goes away in the CL.

Repository Root: http://josm.openstreetmap.de/svn
Build-Date: 2012-05-19 01:31:04
Last Changed Author: bastiK
Revision: 5241
Repository UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
URL: http://josm.openstreetmap.de/svn/trunk
Last Changed Date: 2012-05-18 12:45:10 +0200 (Fri, 18 May 2012)
Last Changed Rev: 5241

Identification: JOSM/1.5 (5241 en)
Memory Usage: 154 MB / 2730 MB (105 MB allocated, but free)
Java version: 1.6.0_32, Sun Microsystems Inc., Java HotSpot(TM) 64-Bit Server VM
Operating system: Windows 7

Plugin: OpeningHoursEditor (27852)
Plugin: buildings_tools (27984)
Plugin: licensechange (27964)
Plugin: mapdust (27884)
Plugin: measurement (27957)
Plugin: openstreetbugs (27852)
Plugin: reverter (28089)
Plugin: turnrestrictions (27891)
Plugin: undelete (28376)
Plugin: utilsplugin2 (28350)

[simon04]

In 5253/josm:

fix #7709 - "Failed to locate image 'null'" after downloading a Map Paint Style