Qt Check if there is an open database connection

[Spitfire]

You're showing wrong part of your code.
Error is generated by the part that executes a query not sets up the database.

You're probably not using connection name there.
When creating the query you have to specify which connection you want to use:

QSqlQuery query( QSqlDatabase::database( "StockManagerConn" ) );

QSqlQuery query( QSqlDatabase::database( "StockManagerConn" ) );

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otherwise it will fall back on the default name and fail.