jTessBoxEditor - Caot Handle 600dpi .png Files

I have a pdf that I have converted to .png at 500dpi and 600dpi. (see below). The 500dpi version works just fine with jTessBoxEditor. But, the 600dpi one fails. I have tried increasing the JVM heap size as suggested here. Even the 600 dpi image is only 91KB. Even when I set the JVM heap size quite large, by ruing jTessBoxEditor as:

java -Xms1024m -Xmx2048m -jar jTessBoxEditorFX.jar

I still fail to be able to open the 600dpi version. Instead, I get a very long set of errors in the terminal. Some snippets of these are:

java.lang.NullPointerException
    at com.sun.javafx.sg.prism.NGCanvas$RenderBuf.validate(NGCanvas.java:213)
    at com.sun.javafx.sg.prism.NGCanvas.initCanvas(NGCanvas.java:640)
    at com.sun.javafx.sg.prism.NGCanvas.renderContent(NGCanvas.java:603)
    at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:2053)
    at com.sun.javafx.sg.prism.NGNode.render(NGNode.java:1945)
    at com.sun.javafx.sg.prism.NGGroup.renderContent(NGGroup.java:235)

...

java.lang.NullPointerException
    at com.sun.javafx.sg.prism.NGCanvas$RenderBuf.validate(NGCanvas.java:213)
    at com.sun.javafx.sg.prism.NGCanvas.initCanvas(NGCanvas.java:640)
    at com.sun.javafx.sg.prism.NGCanvas.renderForcedContent(NGCanvas.java:624)
    at com.sun.javafx.sg.prism.NGGroup.renderForcedContent(NGGroup.java:198)
    at com.sun.javafx.sg.prism.NGGroup.renderForcedContent(NGGroup.java:198)
    at com.sun.javafx.sg.prism.NGGroup.renderForcedContent(NGGroup.java:198)
    at com.sun.javafx.sg.prism.NGGroup.renderForcedContent(NGGroup.java:198)

...

    at com.sun.javafx.sg.prism.NGNode.render(NGNode.java:1945)
    at com.sun.javafx.tk.quantum.ViewPainter.doPaint(ViewPainter.java:477)
    at com.sun.javafx.tk.quantum.ViewPainter.paintImpl(ViewPainter.java:330)
    at com.sun.javafx.tk.quantum.PresentingPainter.run(PresentingPainter.java:91)
    at java.util.concurrent.Executors$RuableAdapter.call(Executors.java:511)
    at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308)
    at com.sun.javafx.tk.RenderJob.run(RenderJob.java:58)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at com.sun.javafx.tk.quantum.QuantumRenderer$PipelineRuable.run(QuantumRenderer.java:125)
    at java.lang.Thread.run(Thread.java:745)

The 500dpi image is here:

and the 600dpi image is here: