In this situation, it is most likely caused by Multicraft's issue with 1.7.x jars.
Note: This only happens if the JAR file doesn't match any entry 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 it matches the selected version.
If any information here is outdated, or if there are still any issues, please create a support ticket.