Fix: Highlighted heading block breaks when moved into a group block

[nani-samireddy]

Closes #56025

What?

As mentioned in #56025, When we add a heading or p tag with a color highlight into a group block. Then It break on the initial render.

Why?

• In the Text-Color Component on this Line, We are using the contentRef.current when we drop heading or p. We might be accessing the element before it is fully rendered. Which gives an undefined value and that undefined is passed down in the functions leading to this error.

How?

• As the contentRef.current is passed down to the functions to calculate styles, Adding a check in the useMemo callback to only call the fillComputedColors function if we have a ref.

Testing Instructions

1. Open the block editor.
2. Add a Heading/Paragraph
3. Add a background highlight color
4. Add group block
5. And drag and drop the Heading/Paragraph into the group block

Testing Instructions for Keyboard

Screenshots or screencast

Before

before.mov

After

after.mov

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