PrefService can throw NullPointerException

[ctrueden]

ImageJ 2.0.0-rc-61/1.51n; Java 1.8.0_112 [64-bit]; Mac OS X 10.12.6; 402MB of 3641MB (11%)
 
java.lang.RuntimeException: java.util.concurrent.ExecutionException: java.lang.NullPointerException
	at net.imagej.legacy.LegacyService.runLegacyCompatibleCommand(LegacyService.java:291)
	at net.imagej.legacy.DefaultLegacyHooks.interceptRunPlugIn(DefaultLegacyHooks.java:163)
	at ij.IJ.runPlugIn(IJ.java)
	at ij.Executer.runCommand(Executer.java:137)
	at ij.Executer.run(Executer.java:66)
	at java.lang.Thread.run(Thread.java:745)
Caused by: java.util.concurrent.ExecutionException: java.lang.NullPointerException
	at java.util.concurrent.FutureTask.report(FutureTask.java:122)
	at java.util.concurrent.FutureTask.get(FutureTask.java:192)
	at net.imagej.legacy.LegacyService.runLegacyCompatibleCommand(LegacyService.java:287)
	... 5 more
Caused by: java.lang.NullPointerException
	at java.util.prefs.AbstractPreferences.put(AbstractPreferences.java:241)
	at org.scijava.prefs.DefaultPrefService$SmartPrefs.put(DefaultPrefService.java:511)
	at org.scijava.prefs.DefaultPrefService.put(DefaultPrefService.java:179)
	at org.scijava.module.DefaultModuleService.save(DefaultModuleService.java:309)
	at org.scijava.module.process.SaveInputsPreprocessor.saveValue(SaveInputsPreprocessor.java:77)
	at org.scijava.module.process.SaveInputsPreprocessor.process(SaveInputsPreprocessor.java:68)
	at org.scijava.module.ModuleRunner.preProcess(ModuleRunner.java:105)
	at org.scijava.module.ModuleRunner.run(ModuleRunner.java:157)
	at org.scijava.module.ModuleRunner.call(ModuleRunner.java:127)
	at org.scijava.module.ModuleRunner.call(ModuleRunner.java:66)
	at org.scijava.thread.DefaultThreadService$3.call(DefaultThreadService.java:238)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	... 1 more