Bismillah.
Shaykh, allama of Yemen, Muqbil ibn Hadi al-Wadi was asked:
ما صحت حديث : ” لولا علي لهلك عمر ” ؟
What is authenticity of hadith “If not Ali, Umar would perish”?
He answered:
هو ما يثبت ، يستشهد به النحويون ، لولا علي لهلك عمر ، لكنه لا يثبت بارك الله فيكم .
It is not proven, grammarian use to cite: If not Ali, Umar would perish. But this (report) isn’t established (proven). Barakallahu fikum.