Currently, the check for the Kernel to be a contract (also not to be the zero address), is in the current Kernel implementation. However, no modules and policies have the logic to ensure this as they inherit from KernelAdapter, which will just set the new kernel without a question. This will work well as long as the new Kernel has the similar logic to check the next Kernels integrity. However, if the logic is forgotten, there is no other safe guard to ensure that the next kernel is not a zero address and is a contract.
Since Kernel is absolutely needed for this systems functionality, there is no possible case that the Kernel should be the zero address. Therefore, it is probably safe to add the checking logic to the KernelAdapter, so every module and policy will check for the next Kernel. It costs more gas since the check is done multiple times, but still arguably it is worth the cost, since Kernel is core part of the system and it will not updated very often.
