Step 3 should be:
Set "Mandatory" to "On" in "General options"

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=19961

Thank you a lot!
But looks like new 9.1.1 of bootstrap-slider.js isnt compatible and cause new bugs. When i've tried to use your fix with old 7.0.1 from the last stable build it was fine.

Works:
mod - application/helpers/qanda_helper.php
mod - application/views/survey/questions/multiplenumeric/rows/sliders/answer_row.php

Cause new bugs:
mod - third_party/bootstrap-slider/bootstrap-slider.js
mod - third_party/bootstrap-slider/bootstrap-slider.min.js

Uploaded some screenshoots with your changes and two versions of bootstrap-slider.js

new 9.1.1 verson of bootstrap-slider.js cause bugs. Need more changes or roll back to the bootstrap-slider.js v. 7.0.1.

so after clarification:

it seems that there the commit :
https://github.com/LimeSurvey/LimeSurvey/commit/9bd89f30f0b890c3208f0a3214af805559337094#commitcomment-18535323

is mixing the update and the hack. As said in the comment, please separate the operations in two commits so we can keep a clear trace of the hack.

If the hack can be avoid: better.

After closer examination and just to clarify, the old 7.1.1 bootstrap slider states that it cannot work with non-numeric values. Which as it seems in not correct.
I did not change a line of the bootstrap-slider.js or -.min.js. All I did was heave it to the latest version and fix our coe to return 'null' on no-input.

I cannot really reproduce the issue, so you would be so kind to show me the output of your debug console, though I could see where the error occurs in your system.

Fresh installation of Limesurvey from Github without any changes. Last commit: https://github.com/LimeSurvey/LimeSurvey/commit/457cdbac944edd72dde297caf245c7cca0963101
9.1.1 version of bootstrap.js
Survey: https://bugs.limesurvey.org/file_download.php?file_id=8318&type=bug

1) Old chrome on win 7 (Chrome/38.0.2125.111) https://aboutmybrowser.com/MvMuXTt2
Console:
bootstrap-slider.js - WARNING: $.fn.slider namespace is already bound. Use the $.fn.bootstrapSlider namespace instead. bootstrap-slider.js:1627
Uncaught TypeError: Cannot read property '0' of null bootstrap-slider.js:767
Failed to load resource: net::ERR_CACHE_MISS http://192.168.100.7:8080/lime-git-160808/index.php/survey/index/sid/368443/newtest/Y/lang/en

2) Yandex browser on win 7 (Chrome/51.0.2704.106 YaBrowser/16.7.0.3342) https://aboutmybrowser.com/wjio0Lp5
Console:
bootstrap-slider.js:1627 bootstrap-slider.js - WARNING: $.fn.slider namespace is already bound. Use the $.fn.bootstrapSlider namespace instead.
bootstrap-slider.js:767 Uncaught TypeError: Cannot read property '0' of null

3) Chrome 52 on win 10 (Chrome/52.0.2743.116) https://aboutmybrowser.com/QJQn4cwY
Console:
bootstrap-slider.js:1627 bootstrap-slider.js - WARNING: $.fn.slider namespace is already bound. Use the $.fn.bootstrapSlider namespace instead.
bootstrap-slider.js:767 Uncaught TypeError: Cannot read property '0' of null

4) Firefox 48 on win 10 https://aboutmybrowser.com/dIQbxS82
Console:
bootstrap-slider.js - WARNING: $.fn.slider namespace is already bound. Use the $.fn.bootstrapSlider namespace instead.bootstrap-slider.js:1627:6
TypeError: this._state.value is null
bootstrap-slider.js:767:6
The Web Console logging API (console.log, console.info, console.warn, console.error) has been disabled by a script on this page.

5) Firefox ESR 45 on win 10 https://aboutmybrowser.com/I2ldzrCm
Console:
bootstrap-slider.js - WARNING: $.fn.slider namespace is already bound. Use the $.fn.bootstrapSlider namespace instead. bootstrap-slider.js:1627:6
TypeError: this._state.value is null
bootstrap-slider.js:767:6
API ведения логов веб-консоли (console.log, console.info, console.warn, console.error) был отключен скриптом на этой странице.

Later I'll try to run LimeSurvey on php 7 instead of 5.4.

And another bug with sliders (i'l make new issue later becouse current one is more important):
If you go to the question options-Advanced settings-Slider and set any value in "Slider initial value" or even set "Slider starts at the middle position" to "On" slider_user_no_action_368443X1X1SQ002 field appear with 0 instead of 1.

https://github.com/seiyria/bootstrap-slider/releases
bootstrap-slider v 9.1.3 works fine, but need small change in CSS file to look like before (move slider handle 5px downside, or just use css from bootstrap-slider repository):

bootstrap-slider.css:
.slider.slider-horizontal .slider-tick,
.slider.slider-horizontal .slider-handle {
margin-left: -10px;
margin-top: -5px; / This string should be removed or set to 0px /
}

This issue can be closed now and i'l make new one about predefined slider position.

I have updated the css to use the latest github version

Version 2.50+ Build 160810 released