If you want to do this specifically with iExplorer, you will have to contact macroplant and obtain support. The program appears to have encountered some sort of error and bugged out. The message you see appears when a C# application attempts to access an object that doesn't exist.
It's basically the equivalent of a null pointer dereference, or
sigsegv in plain C. Figuring out what caused it in a locked-down closed-source obfuscated commercial application (which is what iExplorer appears to be) is basically not worth the effort. Especially since, it turns out, the files you were trying to transfer are simply available to the user on the iDevice. There's no need to use this application.
I've written an answer to the existing question about the topic on how to copy without using iExplorer.