NullPointerException when closing RandomAccessReadMemoryMappedFile more than once

[Matt Bamberger]

I'm using pdfbox 3.0.0-alpha3
When I use a RandomAccessReadMemoryMappedFile to load a PDDocument, I get a 
NullPointerException when I close it after the PDDocument is closed:

try (RandomAccessReadMemoryMappedFile reader = new 
RandomAccessReadMemoryMappedFile(file)) {
  try (PDDocument doc = Loader.loadPDF(reader, null, null, null, null)) {
    // ...
  }
}

I found that it's because this.mappedByteBuffer is already null the second time 
around, so I think you can just wrap in a not-null check:

public void close() throws IOException {
  if (this.fileChannel != null) {
    this.fileChannel.close();
  }

  if (this.mappedByteBuffer != null) { // add this
    Optional.ofNullable(this.unmapper).ifPresent((u) -> {
      u.accept(this.mappedByteBuffer); // exception is thrown in here
    });
    this.mappedByteBuffer = null;
  }
}

Thanks,
Matt