title: Error when removing line with function parameter [jsxgraph]
body: <p>I have a JSXGraph project in which I created a point (A) on the x.axis and a line that is perpendicular to the x-axis and runs through (A). When moving (A) the line should follow the point. That worked fine so far.<br />
  However at the press of a button I would like to remove the line and the point and free the variables that contained the line and the point.<br />
  Here is where my problem lies. After removing the point/line and freeing the variables the update() function still calls the function that was used to define the line, which results in an error, as the variable in the function no longer exists.<br />
  Any idea how to solve this problem?</p>
  <p>Thanks!</p>
  <p>Link to fiddle: <a href="https://jsfiddle.net/t9rcva1x/1/" rel="nofollow noreferrer">https://jsfiddle.net/t9rcva1x/1/</a></p>
  <pre><code>const testBoard = JXG.JSXGraph.initBoard('jxgbox', { axis: true });
  let testPoint = testBoard.create('point', [1, 0]);
  function testfunction() {
      return testPoint.X();
  }
  let testLine = testBoard.create('line', [
      [testfunction, 0],
      [testfunction, 1],
  ]);
  testBoard.removeObject(testLine);
  testBoard.removeObject(testPoint);
  testPoint = null;
  testLine = null;
  testBoard.update();
  </code></pre>