🐛 Fix DeviceSelectorAction NoSuchElementException in the toolbar layout

[AlexV525]

(originally #8496)

This PR fixes a NoSuchElementException that occurs in the DeviceSelectorAction when IntelliJ's toolbar layout system tries to calculate component widths.

Problem

The error manifested as:

java.util.NoSuchElementException: Key io.flutter.actions.DeviceSelectorAction$1[...] is missing in the map.
	at kotlin.collections.MapsKt__MapWithDefaultKt.getOrImplicitDefaultNullable(MapWithDefault.kt:24)
	at kotlin.collections.MapsKt__MapsKt.getValue(Maps.kt:369)
	at com.intellij.openapi.actionSystem.toolbarLayout.CompressingLayoutStrategyKt.calculateComponentWidths(CompressingLayoutStrategy.kt:200)

Root Cause

The getPreferredSize() method in the anonymous JButton class was being called by IntelliJ's layout system before the client properties (ICON_LABEL_KEY, TEXT_LABEL_KEY, ARROW_LABEL_KEY) were set during component initialization. This caused the layout system to fail when trying to register the component in its internal maps because:

1. The method accessed null client properties without proper fallback handling
2. Used unsafe Objects.requireNonNull(fm) calls that could throw exceptions
3. The layout system couldn't determine proper component dimensions during initialization

Solution

Enhanced the getPreferredSize() method with defensive programming:

• Added fallback logic: When client properties are null (during initialization), use the same default icons and text that would normally be used
• Safe null checking: Replaced Objects.requireNonNull(fm) with proper null checks
• Reasonable defaults: Provide sensible sizing estimates using FlutterIcons.Mobile, chevron down icon, and "No device selected" text width

// Before: Unsafe access
width += Objects.requireNonNull(fm).stringWidth(text);

// After: Defensive with fallback
if (fm != null) {
  width += fm.stringWidth(text);
  height = Math.max(height, fm.getHeight());
}

Impact

• Eliminates NoSuchElementException during toolbar initialization
• Maintains exact same functionality once component is fully initialized
• No performance impact - fallback logic only runs during the brief initialization phase
• More robust component that gracefully handles IntelliJ's layout timing

Fixes #8494.