Fixes #426

To test:

• Start the demo app
• Add an image block
• Set an image to it
• Add a caption
• When writing on the caption tap Enter on the end of the caption text
• Check if new paragraph block is inserted
• Do the same test as above using a video block.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.