Language of TM is Undecidable

Question

why is this Problem$$L = \{ \langle M\rangle \mid L(M) \text{ is undecidable}\}$$ undecidable?

I thought if we know $L(M)$ the turingmaschine accepts all $x \in L(M)$, so $L(M)$ is in every case decidable and $L=\emptyset$ especially finite and decidable.

I know we cannot find an algorithm but every TM has a language and we even if we don't it, we know it is decidable.

Can someone help me where I do have a misunderstanding?