Describe the bug
Mantid crashes when you try to remove a function from a composite function inside a convolution function

To Reproduce

1. Load a workspace and open a plot
2. Open the fitting interface
3. Add a convolution function
4. Inside the convolution function add a composite function
5. Inside the convolution function add two functions (e.g. two Gaussians is fine)
6. Attempt to remove one of the Gaussians
   (See screenshot for set up)

Expected behavior
Function should be removed and no crash should occur.

Screenshots

Platform/Version (please complete the following information):

• OS: Recreated on Windows but probably on all OS
• Mantid Version 6.9 - not checked whether it's a regression