Remove an unnecessary null pointer check

[An extra null pointer check is not needed in a function](https://isocpp.org/wiki/faq/freestore-mgmt#delete-handles-null "Do I need to check for null before delete p?") like [the destructor for the class “Viewer”](https://github.com/baidu/ICE-BA/blob/c49560c81e47f6b52b18739194fef89330dbdfa0/Backend/Visualization/Viewer.h#L42 "Update candidate").

Open Backend/Visualization/Viewer.h at line 42 and inspect the Viewer destructor and its null pointer check. Remove the unnecessary check, then verify that the project still builds; done means the destructor no longer performs that redundant check.

Written by the indexing model from the issue text.