When you go to Create_New_World/More_World_Options.../World_Type:Custumized/Custumize/Presets you need to delete all the default text before you can paste (or write) the presets you want.
I suggest you to add a button that deletes all the text.
The same for superflat presets, but in superflat is a small text, so it is not so necessary.
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Even faster: Press CTRL-A, then DEL, works also everywhere.
Anyway, this is not a bug, not even a feature request, but feel free to post it at Minecraft Suggestions on Reddit.
Click on the end of the text, hold down shift, press the home button. It will highlight the whole thing allowing you to easily erase it. This isn't specifically a minecraft thing, all programs do this. Home takes you to the start of the line, end takes you to the end.