Fix a QML error when adding an account

Scenario:

• there is no account yet;
• add a new account

In the console, there is that QML error:

AccountsTab.qml:43:13: QML AccountsComboBox: Binding loop detected for property "implicitWidth"

It was introduced with NXDRIVE-1990 (that commit exactly). As it does not make the GUI weird and all seems fine, this is not critical. But we should fix it still.