FIXED IT!
This is a general-enough fix that I thought I'd post it here.
Open Terminal.app.
Type "ps aux | fgrep Preview.app" and press "Enter".
You will probably see multiple lines, including one that says "fgrep" in it. Ignore that one.
The first column is the process ID. Look for the lowest value. For example, if there are two lines with process IDs of "12289" and "12299," use "12289" in the next step.
Type "kill <processID>" (and Enter), where "<processID>" is the number you chose above. For the example above, you'd type "kill 122289".
You may have to repeat these two steps.
When there are no more "zombie" Preview.app processes, it should be able to be run normally.
Hope this is helpful to someone!