I found a bug from a fresh install of Invoice Ninja via Softaculous that can cause a Whoops error right off the bat using version 4.5.19. The error occurs on /app/Models/Account.php
and the current line of code is:
$currencyId = ($client && $client->currency_id) ? $client->currency_id : $this->currency_id ?: DEFAULT_CURRENCY;
which is currently deprecated. You can fix this by changing the line to the new format:
$currencyId = ($client && $client->currency_id) ? $client->currency_id : ($this->currency_id ?: DEFAULT_CURRENCY);
This fixed the error for me!