When running for President there are few topics that are off-limits. Donald Trump cannot avoid questions about racism if he wants to be president. There are many many topics that he does not have to talk about as a hotel owner and entertainment personality, but almost nothing is off limits in a presidental race.
Some issues should not be off the table if you are a widely known public figure and happen to be running for President of the United States of America. Donald Trump would presumably be governing people of all races, creeds and religions as they make up America's people. So no, he shouldn't get a pass on questions about racism or sexism.
The president of the United States represents all people and the issues faces by everyone in the country. The presidential candidates must be able to answer all questions and speak to their convictions on issues. The more controversial the topic, the more important it is that presidential candidates voice their opinion.
Politicians should not be pampered by being allowed to evade questions about thorny and sensitive issues such as racism. Particularly if a person aspires to be the president, they should have a clear view of what they will do to address this kind of problem and potential voters have the right to know exactly what it is. Journalists should be under no pressure to avoid sensitive questions to save the candidate from embarrassment.