Overlay assertion error if both color and blur are specified.

[pranavkumark13]

Describe the bug:

I tried setting both color and blur to the property overlayStyle as mentioned in the documentation, but I am facing an assertion error saying "'color== null || blur == null': is not true." In the example code provided, the color property in the overlayStyle is commented out. There is a requirement for us to have both color and blur.

To Reproduce:

Steps to reproduce the behavior:
Add this

overlayStyle: ExpandableFabOverlayStyle(
          color: Colors.black.withOpacity(0.5),
          blur: 5,
        ),

to the ExpandableFab widget.

Expected Behavior:

Overlaystyle should work by adding both color and blur. The Background of the fab button should have the color which is passed.