ImgSaver can throw NPE

[ctrueden]

• Launch Fiji
• Shift+B for Blobs
• File > Export > Image...
• blobs.ims

The following exception appears:

java.lang.RuntimeException: java.util.concurrent.ExecutionException: java.lang.NullPointerException
    at net.imagej.legacy.DefaultLegacyService.runLegacyCompatibleCommand(DefaultLegacyService.java:256)
    at net.imagej.legacy.DefaultLegacyHooks.interceptRunPlugIn(DefaultLegacyHooks.java:160)
    at ij.IJ.runPlugIn(IJ.java)
    at ij.Executer.runCommand(Executer.java:131)
    at ij.Executer.run(Executer.java:64)
    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.DefaultLegacyService.runLegacyCompatibleCommand(DefaultLegacyService.java:252)
    ... 5 more
Caused by: java.lang.NullPointerException
    at io.scif.img.ImgSaver.writePlanes(ImgSaver.java:589)
    at io.scif.img.ImgSaver.writeImg(ImgSaver.java:541)
    at io.scif.img.ImgSaver.writeImg(ImgSaver.java:527)
    at io.scif.img.ImgSaver.writeImg(ImgSaver.java:497)
    at io.scif.img.ImgSaver.writeImg(ImgSaver.java:459)
    at io.scif.img.ImgSaver.saveImg(ImgSaver.java:174)
    at io.scif.img.ImgSaver.saveImg(ImgSaver.java:158)
    at io.scif.services.DefaultDatasetIOService.save(DefaultDatasetIOService.java:154)
    at io.scif.services.DefaultDatasetIOService.save(DefaultDatasetIOService.java:142)
    at net.imagej.plugins.commands.io.SaveAsImage.run(SaveAsImage.java:93)
    at org.scijava.command.CommandModule.run(CommandModule.java:201)
    at org.scijava.module.ModuleRunner.run(ModuleRunner.java:167)
    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)

[hinerm]

The ImarisWriter NPE is fixed in this commit.