PR: Don't show TextEditor if it fails to be initialized

[mariacamilarg]

Fixes #3673

[ccordoba12]

@mariacamilaremolinagutierrez, this is not the solution I asked you to implement. What I want is for you to add a method called setup_and_check to the TextEditor class, and use it to verify if it's able to display strings passed to it. setup_and_check must return True is there are no problems and False otherwise.

Besides, please add a test for that method using the code I put on issue #3673 in

spyder/widgets/variableexplorer/tests/test_texteditor.py

[mariacamilarg]

@ccordoba12 This is ready :)