Describe the bug
Found in #3771

When data is loaded but no models are chosen, one can still click on Add Constraints button, leading to unsightly exceptions being thrown.

To Reproduce
Steps to reproduce the behavior:

1. Open the Const/Simul Fit Window
2. Load at least two data sets and send both to fitting.
3. The Add constraints is now active in the Const/Simul Window. Press it!
4. See the error

Expected behavior
The Add Constraints button should be disabled when no model is available
The message should be meaningful rather than a simple exception

SasView version (please complete the following information):

• Version: 6.1.1

Operating system (please complete the following information):

• OS: Windows 10

Installation method (please complete the following information):

• Installed: dev