There was nothing racist about Trump’s tweet. It was stupid and ignorant, yes, just like his reference to the revolutionaries taking over the airports, but he did not mention race at all. Anyone who calls it racist is merely projecting their own racism on him.
And it was not stupid to tell these women that if they don’t like America they should go somewhere else. The only stupid thing was that he told them all to go back where they came from, not realizing that only one was an immigrant. The most reasonable explanation is that he simply forgot he was addressing four individuals, and those words were directed only to Omar, who should indeed go back to Somalia rather than trying to turn America into Somalia.