If you are using the Bootswatch themes for your project, these are only compatible with Mendix 5.0 modeller. If you are using Mendix 4 you should use a Mendix 4 compatible theme. The bootstrap module for Mendix 4 is available on the appstore and compatible for Mendix 4.
i have tried by creating new project but it is still showing the same error would it help if i reinstall the web browser..?