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Sitecore error “Server cannot modify cookies after HTTP headers have been sent.”

I am trying to Insert a new form. When I am trying to select a placeholder for the form, "Sitecore error "Server cannot modify cookies after HTTP headers have been sent" error is showing.
First i installed WFFM package and then followed by Powershell and SXA. Im getting this error after installing SXA package. Im using Sitecore 8.2 Update 1.
 
Any suggestion what to do?