In this situation, it is most likely caused because of Multicraft's issue with 1.7.x jars.
Note: It only happens if the JAR file doesn't match anything in the JAR Selection menu.
To prevent this from happening, select one of the JAR files from the Minecraft JAR selection menu. For a custom JAR, ensure the JAR file matches the selected version.
If any information you see here is outdated, or you are still experiencing issues, please let us know by creating a support ticket. Thank you.