[Bug] Wallet is stuck at "Preparing transaction..." if too many shielded UTXOs are being selected

[ambassador000]

Tried on on testnet5: By selecting many shielded UTXOs (more than 1000) and trying to send them all in 1 transaction to 1 transparent address, wallet will enter the "Preparing transaction..." phase and it will look like it is processing the transaction, but in reality wallet is stuck and nothing will change even after 30+ minutes of waiting. User doesn't have anything else to do but force shutdown the wallet while it's in the middle of the transaction creation process.

After quitting the wallet, the terminal on Ubuntu 20.04 outputs the following:

pivx-qt: /usr/include/boost/optional/optional.hpp:1217: boost::optional<T>::pointer_type boost::optional<T>::operator->() [with T = QString; boost::optional<T>::pointer_type = QString*]: Assertion this->is_initialized()' failed.`

[furszy]

Thanks for making an issue for this.

Spend notes is an expensive task, create a transaction with +1k spends can definitely take a while.
In the future (post v6 when we are calmer..), can add a progress bar to the transaction creation process and a "stop/cancel" functionality as well.
At the end, this isn't something that will affect regular users anyway, only a "nice-to-have" functionality.