boolean isValidInput() {
//		if (isUserDefined && currentFallthroughAddr == null) {
//			Instruction inst = program.getListing().getInstructionContaining(currentAddr);
//			if (inst.getFallThrough() != null) {
//				return false;
//			}
//		}
		return true;
	}
The option for the "user defined" fallthrough was chosen.
Ghidra/Features/Base/src/main/java/ghidra/app/plugin/core/fallthrough/FallThroughModel.java