In some cases, a program which cannot be quit during normal operation (like the Finder or the Dock) could cause a technical problem with your computer. Such a problem becomes even more severe if automatic login of a user is active, so the erroneous application is becoming active by itself after each startup. To resolve such a problem by using a second user account, the automatic login of a user after startup can be switched off by TinkerTool System for Recovery Mode.
Automatic login can be reenabled later if desired, by selecting System Settings > Users & Groups > Automatically log in as… in macOS.