Jalview Develop Launcher fails if it used to launch a Jalview release channel

To reproduce - try to launch the Jalview release JVL with the Jalview Develop launcher.

On launching a second time, a tiny window with no content is shown (presumably the splashscreen) - looking at the logs (below) it seems the splashscreen display fails because the images can't be loaded.

2021/06/10 09:37:39:237 INFO ------------------ VM Info ------------------
2021/06/10 09:37:39:237 INFO -- OS Name: Mac OS X
2021/06/10 09:37:39:237 INFO -- OS Arch: x86_64
2021/06/10 09:37:39:238 INFO -- OS Vers: 10.16
2021/06/10 09:37:39:238 INFO -- Java Vers: 11.0.9
2021/06/10 09:37:39:238 INFO -- Java Home: /Applications/Jalview Develop.app/Contents/Resources/app/jre/Contents/Home
2021/06/10 09:37:39:242 INFO -- Install4j Vers: null
2021/06/10 09:37:39:242 INFO -- Install4j Template Vers: 2.11.2.0-d20201215_F2b2fb34c_C5d501d5230
2021/06/10 09:37:39:243 INFO -- User Name: jprocter
2021/06/10 09:37:39:243 INFO -- User Home: /Users/jprocter
2021/06/10 09:37:39:243 INFO -- Cur dir: /Applications/Jalview Develop.app
2021/06/10 09:37:39:243 INFO -- Launcher version: 1.8.3-1.2.10_JVL
2021/06/10 09:37:39:243 INFO -- startupFilesParameterString:
2021/06/10 09:37:39:243 INFO ---------------------------------------------
2021/06/10 09:37:39:258 INFO Getdown starting [version=1.8.3-1.2.10_JVL, built=2020-12-01 14:54]
2021/06/10 09:37:39:263 INFO Checking whether we need to use a proxy...
2021/06/10 09:37:39:498 INFO Keep on top set to [keep_on_top=true]
2021/06/10 09:37:39:504 WARNING Failed to load image [path=/Applications/Jalview Develop.app/Contents/Resources/app/resource/JalviewLogo_Huge.png, error=javax.imageio.IIOException: Can't read input file!]
2021/06/10 09:37:39:504 WARNING Error loading icon image [path=resource/JalviewLogo_Huge.png]
2021/06/10 09:37:39:504 WARNING Failed to load any icons [iconImages=[resource/JalviewLogo_Huge.png]]
2021/06/10 09:37:39:504 WARNING Failed to load image [path=/Applications/Jalview Develop.app/Contents/Resources/app/resource/jalview_logo_background_getdown_instant-640x480.png, error=javax.imageio.IIOException: Can't read input file!]
2021/06/10 09:37:39:504 WARNING Failed to load image [path=/Applications/Jalview Develop.app/Contents/Resources/app/resource/jalview_logo_background_getdown-640x480.png, error=javax.imageio.IIOException: Can't read input file!]
2021/06/10 09:37:39:511 WARNING Tried to invokeAndWait but couldn't. Going to invokeLater instead [Exception=null]
2021/06/10 09:37:39:630 WARNING Failed to load image [path=/Applications/Jalview Develop.app/Contents/Resources/app/resource/jalview_logo_background_getdown-640x480.png, error=javax.imageio.IIOException: Can't read input file!]
2021/06/10 09:37:39:631 WARNING Failed to load image [path=/Applications/Jalview Develop.app/Contents/Resources/app/resource/jalview_logo_background_getdown-progress.png, error=javax.imageio.IIOException: Can't read input file!]
2021/06/10 09:37:39:632 INFO Downloading 0 resources [totalBytes=0, maxConcurrent=10]
2021/06/10 09:37:39:635 INFO Testing whether proxy is needed, via: https://builds.jalview.org/browse/JB-GPC235/latest/artifact/shared/getdown-channel/1.8/getdown.txt
2021/06/10 09:37:40:071 INFO No proxy appears to be needed.
2021/06/10 09:37:40:072 INFO Downloading 0 resources [totalBytes=0, maxConcurrent=10]
2021/06/10 09:37:40:075 INFO Able to lock for updates: true
2021/06/10 09:37:40:076 INFO Verifying application: https://builds.jalview.org/browse/JB-GPC235/latest/artifact/shared/getdown-channel/1.8/
2021/06/10 09:37:40:076 INFO Version: -1
2021/06/10 09:37:40:076 INFO Class: jalview.bin.Jalview
2021/06/10 09:37:40:077 INFO Attempting to refetch 'digest.txt' from 'https://builds.jalview.org/browse/JB-GPC235/latest/artifact/shared/getdown-channel/1.8/digest.txt'.
2021/06/10 09:37:40:131 INFO No signing certs, not verifying digest.txt [path=digest.txt]
2021/06/10 09:37:40:132 INFO Attempting to refetch 'digest2.txt' from 'https://builds.jalview.org/browse/JB-GPC235/latest/artifact/shared/getdown-channel/1.8/digest2.txt'.
2021/06/10 09:37:40:186 INFO No signing certs, not verifying digest.txt [path=digest2.txt]
2021/06/10 09:37:40:504 INFO Verified resources [count=59, size=20369k, duration=316ms]
2021/06/10 09:37:40:506 INFO Checking Java version [current=11000900, wantMin=1080000, wantMax=1089999]
2021/06/10 09:37:40:506 INFO Attempting to update Java VM...
2021/06/10 09:37:40:506 WARNING getdown() failed.
java.io.IOException: m.java_download_failed
        at com.threerings.getdown.launcher.Getdown.updateJava(Getdown.java:502)
        at com.threerings.getdown.launcher.Getdown.getdown(Getdown.java:382)
        at com.threerings.getdown.launcher.Getdown.run(Getdown.java:150)
2021/06/10 09:37:40:507 INFO Releasing lock