Power Query - Create new column based on latest date in two other columns

Hi again , 

try to add a condition in the begin to check if the signature date is null 

someting like that: 

if
[Opportunity History.First Signature Date] = null
then [Start_date__c]
else
if
[Start_date__c] >= [Opportunity History.First Signature Date]
then [Start_date__c]
else
if
[Opportunity History.First Signature Date] = "null"
then [Start_date__c]
else
[Opportunity History.First Signature Date]

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Best Regards
BC