Fix hierarchy scroll-to for mobile situation

[evnchn]

Motivation

While working on another PR, I noticed that the NiceGUI documentation kept throwing errors in the console, and I realized that the scroll-into-view functionality never worked for mobile users, since the DOM is not populated until the tree is expanded.

Implementation

• Use ?. such that it doesn't throw an error, instead fails silently.
• Run the scroll-into-view on toggling the left drawer.

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
• Pytests have been added (or are not necessary).
• Documentation has been added (or is not necessary).

Results

Before:

-> Wrong scroll position, errors thrown

After:

-> Correct scroll position, no errors