pct2rgb.py: "AttributeError: 'NoneType' object has no attribute 'GetCount'" error if the color table is missing in the input raster

[agiudiceandrea]

Expected behavior and actual behavior.

In case pct2rgb.py is executed against a source raster file without a color table, then the following Python error is displayed:

F:\QGIS\OSGEO4~2.3\qgis\apps\Python39\lib\site-packages\osgeo\gdal.py:290: FutureWarning: Neither gdal.UseExceptions() nor gdal.DontUseExceptions() has been explicitly called. In GDAL 4.0, exceptions will be enabled by default. 
warnings.warn(
Traceback (most recent call last):
File "F:\QGIS\OSGEO4~2.3\qgis\apps\Python39\lib\site-packages\osgeo_utils\auxiliary\gdal_argparse.py", line 175, in main 
self.doit(**kwargs)
File "F:\QGIS\OSGEO4~2.3\qgis\apps\Python39\lib\site-packages\osgeo_utils\pct2rgb.py", line 220, in doit 
return pct2rgb(**kwargs)
File "F:\QGIS\OSGEO4~2.3\qgis\apps\Python39\lib\site-packages\osgeo_utils\pct2rgb.py", line 88, in pct2rgb 
ct_size = ct.GetCount() 
AttributeError: 'NoneType' object has no attribute 'GetCount'

I think it would be better to gracefully handle that case and raise an exception.

Operating system

Windows 10 64 bit

GDAL version and provenance

GDAL 3.7.3 from OSGeo4W