Fix: guard against null line_model in distance.element position setter

[xianjianlf2]

Summary

Fixes #686.

The position setter in app/components/selection/distance.element.js passed line_model directly into the styleProps setter and render(), both of which destructure it ({y, x, d, q, v, color} and {q, d} respectively).

When line_model is null or undefined — which can happen during rapid mouse movement or when measured elements dynamically unmount in single-page apps — destructuring throws a TypeError and crashes the measurement overlay.

Change

Add an early-return guard at the top of the position setter so a falsy line_model is ignored instead of crashing:

set position({line_model, node_label_id}) {
  if (!line_model) return

  this.styleProps = line_model
  this.$shadow.innerHTML = this.render(line_model, node_label_id)
}

Minimal, no behavior change for valid input.

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