Error while generating heighmap

[batchku]

Hello,
Thank you for this plugin.

I'm trying to use it with QGIS 3.14.
I have a vector polygon layer with a rectangular feature selected, and I attempt to run the plugin via

Raster > Heighmap Export > Heighmap Export

This is what I see in the python error console:

2020-08-29T21:08:40     WARNING    Traceback (most recent call last):
              File "/Users/ali/Library/Application Support/QGIS/QGIS3/profiles/default/python/plugins/HeightmapExport/heightmapexport.py", line 69, in run
              hmexport = HeightmapExportDialog(self.iface)
              File "/Users/ali/Library/Application Support/QGIS/QGIS3/profiles/default/python/plugins/HeightmapExport/heightmapexport_dialog.py", line 96, in __init__
              self.layer_changed(layer)
              File "/Users/ali/Library/Application Support/QGIS/QGIS3/profiles/default/python/plugins/HeightmapExport/heightmapexport_dialog.py", line 311, in layer_changed
              self.get_dimensions(layer)
              File "/Users/ali/Library/Application Support/QGIS/QGIS3/profiles/default/python/plugins/HeightmapExport/heightmapexport_dialog.py", line 368, in get_dimensions
              (zmin, zmax) = src.GetRasterBand(1).ComputeRasterMinMax(False)
             AttributeError: 'NoneType' object has no attribute 'GetRasterBand'