Otherwise, if 'stack' folder exists but is empty, the script will return ERROR as below:
```
/net/kraken/nobak/zyunjun/TonopahSenAT64/stack already exists.
Replacing master with existing stack.
updating the valid overlap region of:
/net/kraken/nobak/zyunjun/TonopahSenAT64/stack
******************
swath: 1
Traceback (most recent call last):
File "/home/zyunjun/tools/isce2/contrib/stack/topsStack/extractCommonValidRegion.py", line 109, in <module>
main()
File "/home/zyunjun/tools/isce2/contrib/stack/topsStack/extractCommonValidRegion.py", line 95, in main
topMaster = ut.loadProduct(os.path.join(inps.master , 'IW{0}.xml'.format(swath)))
File "/home/zyunjun/tools/isce2/contrib/stack/topsStack/s1a_isce_utils.py", line 26, in loadProduct
obj = pm.loadProduct(xmlname)
File "/net/kraken/home1/geomod/apps/rhel7/isce2-github/isce/components/iscesys/Component/ProductManager.py", line 25, in loadProduct
self.load(filename)
File "/net/kraken/home1/geomod/apps/rhel7/isce2-github/isce/components/iscesys/Component/Configurable.py", line 1407, in load
tmpProp, tmpFact, tmpMisc = FP.parse(filename)
File "/net/kraken/home1/geomod/apps/rhel7/isce2-github/isce/components/iscesys/Parsers/XmlParser.py", line 41, in parse
root = ET.parse(filename)
File "/net/kraken/home1/geomod/apps/anaconda3/lib/python3.7/xml/etree/ElementTree.py", line 1197, in parse
tree.parse(source, parser)
File "/net/kraken/home1/geomod/apps/anaconda3/lib/python3.7/xml/etree/ElementTree.py", line 587, in parse
source = open(source, "rb")
FileNotFoundError: [Errno 2] No such file or directory: '/net/kraken/nobak/zyunjun/TonopahSenAT64/stack/IW1.xml'
```
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