That looks like a weird bug that occurs when deleting entities in your project.
Does this happen with every single one of your projects, or just one specific one? Which TBC version do you use. How large is the project, could you upload the VCE here.
Every drawing entity has an internal serial number, which the user never gets to see.
If you join lines the geometric information needs to be transferred to one common entity. All the now empty entities need to be deleted from the project, and all references to them (i.e. is it a surfaces breakline) need to be changed to the combined linestring. If something fails here along the way that may lead to that error you see. It looks as if the program tries to access an entity that's not there any longer.
You should contact your dealer and have Trimble investigate your project and why this is happening.
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Ronny Schneider
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