DefaultPrefService can throw NPE

[ctrueden]

The following exception can be thrown by the DefaultPrefService when persisting values:

java.lang.NullPointerException
    at java.util.prefs.AbstractPreferences.put(AbstractPreferences.java:241)
    at org.scijava.prefs.DefaultPrefService.put(DefaultPrefService.java:183)
    at org.scijava.module.DefaultModuleService.save(DefaultModuleService.java:280)
    at org.scijava.module.process.SaveInputsPreprocessor.saveValue(SaveInputsPreprocessor.java:74)
    at org.scijava.module.process.SaveInputsPreprocessor.process(SaveInputsPreprocessor.java:65)
    at org.scijava.module.ModuleRunner.preProcess(ModuleRunner.java:104)
    at org.scijava.module.ModuleRunner.run(ModuleRunner.java:156)
    at org.scijava.module.ModuleRunner.call(ModuleRunner.java:126)
    at org.scijava.module.ModuleRunner.call(ModuleRunner.java:65)
    at org.scijava.thread.DefaultThreadService$2.call(DefaultThreadService.java:164)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

To reproduce, open the SingleInputPreprocessorTest, remove the persist = false attribute from the private T thing parameter of HasASingleInput, and run the tests.

[ctrueden]

This is probably a SciJava Common bug, but the SingleInputPreprocessorTest provides an easy way to reproduce, and that class lives in ImageJ Common.