Recently we come to know with one of the known issue that if you install latest SearchStax module with Sitecore https://www.searchstax.com/docs/searchstudio/sitecore-module-2/ and then after creating new app using SearchStax Configuration module page.
Once new app created and then try to run Populate schema, will get solr connectivity exception.
Currently if you are facing such issue then
Please get latest hotfix Sitecore SearchStax official module. Directly installing will help to fix this issue.