Instruction Set does not raise error when adding invalid instruction

It seems there are no checks when adding / editing an instruction. The instructions set view shows the invalid instructions but the model does not use them. Therefore, we can add an instruction inst with an already existing pair of icode/ifun, see it in the instruction set and get a compilation error, saying inst does not exist. We never faced the bug before, it's likely a regression from the issue #2 (closed).