When trying to enable Character Viewer on 10.9.5, it does not show up,
and I get a message:

2017-03-07 14:33:20.008 java[57089:507] [IMKInputSession activate] exception caught.
IMKInputSession: <IMKInputSession: 0x7fdedd598d00> --
NSInvalidArgumentException : [NOTE: this exception originated in the server.]
*** setObjectForKey: key cannot be nil

To setup for this:
System Preferences -> Keyboard -> Show Keyboard and Characters Viewers
should be checked.
Then there should be a Flag on the system bag (which is the input selection pulldown)

Pull up TextEdit (easy way, Command+space, TextEdit)

Focus on TextEdit and Click the flag, then Show Character Viewer. It should come up, and allow you to stuff odd characters like emoji into the TextEditor.

Now try it with a sample JFX app with a text field:
java -cp Hello/dist/Hello.jar hello.HelloTextArea
and try it again. The above message will be shown on console and the viewer will not appear.