https://github.com/bitcoin/bitcoin/blob/9ad6f14175c19b5d47267c3dd5840c14bad43c83/src/validation.cpp#L3711

If i'm following right the flow of the code, when we hit this lane `pindex` should be always equal to nullptr thus this check may be omitted
AssertLockHeld(cs_main);
    
    uint256 hash = block.GetHash();
    BlockMap::iterator it = m_block_index.find(hash);
    if (it != m_block_index.end())
        return it->second;
    
    CBlockIndex* pindexNew = new CBlockIndex(block);
    
    
    
    pindexNew->nSequenceId = 0;
    BlockMap::iterator mi = m_block_index.insert(std::make_pair(hash, pindexNew)).first;
    pindexNew->phashBlock = &((*mi).first);
    BlockMap::iterator miPrev = m_block_index.find(block.hashPrevBlock);
    if (miPrev != m_block_index.end())
    {
        pindexNew->pprev = (*miPrev).second;
        pindexNew->nHeight = pindexNew->pprev->nHeight + 1;
        pindexNew->BuildSkip();
    }
    pindexNew->nTimeMax = (pindexNew->pprev ? std::max(pindexNew->pprev->nTimeMax, pindexNew->nTime) : pindexNew->nTime);
    pindexNew->nChainWork = (pindexNew->pprev ? pindexNew->pprev->nChainWork : 0) + GetBlockProof(*pindexNew);
    pindexNew->RaiseValidity(BLOCK_VALID_TREE);
    if (pindexBestHeader == nullptr || pindexBestHeader->nChainWork < pindexNew->nChainWork)
        pindexBestHeader = pindexNew;
    setDirtyBlockIndex.insert(pindexNew);
    return pindexNew;
}
void CChainState::ReceivedBlockTransactions(const CBlock& block, CBlockIndex* pindexNew, const FlatFilePos& pos, const Consensus::Params& consensusParams)