While running laravel phpunit testcase , i'm getting an error "Call to a member function connection() on null" in Model.php line 1249

i'm running laravel phpunit testcase, getting error in DB connection. DB connected successfully. But, "Call to a member function connection() on null" - Could not open to database connection server. Please update the configuration settings. I did all those things properly. Find below details,

"php" : ">=7.1.3", "laravel/lumen-framework": "5.8.", "phpunit/phpunit" : "^7.0", "phpunit/php-invoker" : "", "phpunit/dbunit" : "^4.0",