p5.SceneManager-error_showPreviousScene()-functionality

error.txt

                                    p5@1.1.9.js:46032 
🌸 p5.js says: There's an error as "showPreviousScene" could not be called as a function (on line 36 in sketch.js [http://127.0.0.1:5501/p5/__scenes/sketch.js:36:18]).
Verify whether "sceneManager" has "showPreviousScene" in it and check the spelling, letter-casing (Javacript is case-sensitive) and its type.
For more: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Errors/Not_a_function#What_went_wrong

                                    p5@1.1.9.js:46374
▶️ Error at line 36 in "playtestKeyPressed" in sketch.js (http://127.0.0.1:5501/p5/__scenes/sketch.js:36:18)

                                    sketch.js:36
▶️ Called from line 135 in "scene1.keyPressed" in sketch.js (http://127.0.0.1:5501/p5/__scenes/sketch.js:135:5)

▶️ Uncaught TypeError: Cannot read property 'fnScene' of undefined
    at SceneManager.showPreviousScene (p5.scenemanager@2018.06.19.js:168)
    at playtestKeyPressed (sketch.js:36)
    at scene3.keyPressed (sketch.js:180)
    at SceneManager.handleEvent (p5.scenemanager@2018.06.19.js:211)
    at o.<computed> (p5.scenemanager@2018.06.19.js:48)
    at p5._main.default._onkeydown (p5@1.1.9.js:62677)