Handle falsy parameters to startExercise

}

/**

 * Start an exercise, also updating the currentState

 * Start an exercise, also updating the currentState.

 *

 * If no exercise is given, none is started. In this case the function only has 

 * the effect of marking all exercises as not-current (i.e. collapsing them).

 * @param {Exercise} exercise Exercise to start

 */

function startExercise(exercise) {

    // Display all exercises as non-current, except for this one

    document.querySelectorAll(".exercise").forEach(el => displayAsNonCurrent(el))

    if(!exercise) return

    const cardEl = document.querySelector(`.exercise[data-exercise-index='${exercise.index}']`)

    let unusuallyLongTimeoutId = null

    )

    exercise.start()

    // Display all exercises as non-current, except for this one

    document.querySelectorAll(".exercise").forEach(el => displayAsNonCurrent(el))

    displayAsCurrent(cardEl)

}