MATLAB does not generally support indexing** directly into the output of functions or operations. The usual solution is to allocate to a temporary variable. If you want this feature then please submit an enhancement request.
You could use subsref to do this without a temporary variable, but it would be obfuscated and complex code that you will later regret writing. I do not recommend doing this.
** Version R2019b and later allows dot indexing into function outputs, see:
