The character 'a' isn't valid data for plot(). You might mean
plot(app.UIAxes,a)
... but passing an image to plot() isn't likely to do anything but create a bunch of nonsense.
That said, you're already displaying the image a by doing
imshow(a);
... so I don't see why the plot() call is even necessary. You could perhaps be more explicit with imshow():
imshow(a,'parent',app.UIAxes);
As an aside, I don't know why you're setting the 'filename' field to a. Why would a filename be image data? Maybe you meant
setappdata(0,'filename',filename)
or
setappdata(0,'filename',fullfilename)
