Enabling icon preview of non-standard icon packages hangs intellij

## Steps to Reproduce
Add font_awesome_flutter to Font packages, press Apply and OK, doing so hangs Intellij forever and afterwards re-opening the project hangs as well, at the bottom of Intellij Checking Icons appears, but it never completes, consuming 90+% CPU on an Intel i7.

The feature was probably developed in: https://github.com/flutter/flutter-intellij/pull/5595

## Version info

[√] Flutter (Channel stable, 2.5.3, on Microsoft Windows [Version 10.0.19043.1288], locale en-US)
• Flutter version 2.5.3 at D:\Apps\flutter
• Upstream repository https://github.com/flutter/flutter.git
• Framework revision 18116933e7 (3 weeks ago), 2021-10-15 10:46:35 -0700
• Engine revision d3ea636dc5
• Dart version 2.14.4

[√] Android toolchain - develop for Android devices (Android SDK version 31.0.0)
• Android SDK at D:\Apps\Android\Sdk
• Platform android-31, build-tools 31.0.0
• ANDROID_HOME = D:\Apps\Android\Sdk
• Java binary at: D:\Apps\Android Studio\jre\bin\java
• Java version OpenJDK Runtime Environment (build 11.0.10+0-b96-7249189)
• All Android licenses accepted.

[√] Chrome - develop for the web
• Chrome at C:\Program Files (x86)\Google\Chrome\Application\chrome.exe

[√] Android Studio (version 2020.3)
• Android Studio at D:\Apps\Android Studio
• Flutter plugin can be installed from:
https://plugins.jetbrains.com/plugin/9212-flutter
• Dart plugin can be installed from:
https://plugins.jetbrains.com/plugin/6351-dart
• Java version OpenJDK Runtime Environment (build 11.0.10+0-b96-7249189)

[√] IntelliJ IDEA Ultimate Edition (version 2021.1)
• IntelliJ at D:\Apps\JetBrains\IntelliJ IDEA 2021.1
• Flutter plugin can be installed from:
https://plugins.jetbrains.com/plugin/9212-flutter
• Dart plugin can be installed from:
https://plugins.jetbrains.com/plugin/6351-dart

[√] Connected device (2 available)
• Chrome (web) • chrome • web-javascript • Google Chrome 95.0.4638.69
• Edge (web) • edge • web-javascript • Microsoft Edge 94.0.992.38

• No issues found!