[POW][Bug] Mining does not work

[inshakovav]

Summary:
The Wallet app (pivx-qt) gives the mining error after a restart.

Steps to Reproduce:

1. Start the wallet pivx-qt
2. Start mining using the command:
   setgenerate true
3. Restart the pivx-qt
4. Start mining again.

Expected Results:
Generate next block

Actual Results:
We get the error: "ERROR: PIVXMiner : generated block is stale"

Most likely the error appeared after the commit:
furszy committed on 17 Dec 2019
4976c0a

Perhaps, after the wallet reboots, the variable
g_best_block in file src/miner.cpp
is not correctly initialized at startup.

[random-zebra]

PoW phase has ended both on mainnet and testnet. Only staking allowed now.
You can mine on regtest with the generate RPC.