I solved the same issue by following steps:
Check the angular version: Using command: ng version My angular version is: Angular CLI: 7.3.10
After that I have support version of ngx bootstrap from the link: https://www.npmjs.com/package/ngx-bootstrap
In package.json file update the version: "bootstrap": "^4.5.3", "@ng-bootstrap/ng-bootstrap": "^4.2.2",
Now after updating package.json, use the command npm update
After this use command ng serve and my error got resolved