J78246 - I observe that same error message if I launch the GUI by opening the .fig file. When you do this, only the figure with the GUI controls appears. None of the normal or needed initializations occur, so the structure handles is not a structure. It doesn't even exist.
You can launch a GUIDE created GUI by calling the m file from the Command Window as
where myGui corresponds to the m and fig files named myGui.m and myGui.fig respectively. You can also launch the GUI by pressing the green run button in the GUI figure editor or by pressing the green run button in the MATLAB m-file editor.
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