Fix get_class_icon() ignoring fallback

[KoBeWi]

Fixes crash introduced by #106866

CrashHandlerException: Program crashed
Engine version: Godot Engine v4.5.dev.custom_build (b89c47bb8511433440e5e547afe4459ca3383090)
Dumping the backtrace. Please include this when reporting the bug on: https://github.com/godotengine/godot/issues
[0] EditorHelp::_add_type_icon (C:\godot_source\editor\editor_help.cpp:471)
[1] EditorHelp::_add_type_icon (C:\godot_source\editor\editor_help.cpp:471)
[2] EditorHelp::_update_doc (C:\godot_source\editor\editor_help.cpp:1015)
[3] EditorHelp::_goto_desc (C:\godot_source\editor\editor_help.cpp:695)
[4] EditorHelp::_request_help (C:\godot_source\editor\editor_help.cpp:2312)
[5] EditorHelp::_help_callback (C:\godot_source\editor\editor_help.cpp:2328)
[6] EditorHelp::go_to_help (C:\godot_source\editor\editor_help.cpp:3263)
[7] ScriptEditor::_help_class_goto (C:\godot_source\editor\plugins\script_editor_plugin.cpp:3705)
[8] call_with_variant_args_helper<ScriptEditor,String const &,0> (C:\godot_source\core\variant\binder_common.h:223)
[9] call_with_variant_args<ScriptEditor,String const &> (C:\godot_source\core\variant\binder_common.h:338)
[10] CallableCustomMethodPointer<ScriptEditor,void,String const &>::call (C:\godot_source\core\object\callable_method_pointer.h:107)
[11] Callable::callp (C:\godot_source\core\variant\callable.cpp:58)
[12] Object::emit_signalp (C:\godot_source\core\object\object.cpp:1288)
[13] Node::emit_signalp (C:\godot_source\scene\main\node.cpp:4315)
[14] Object::emit_signal<Variant> (C:\godot_source\core\object\object.h:934)
[15] EditorHelpSearch::_confirmed (C:\godot_source\editor\editor_help_search.cpp:202)
[16] call_with_variant_args_helper<EditorHelpSearch> (C:\godot_source\core\variant\binder_common.h:228)
[17] call_with_variant_args<EditorHelpSearch> (C:\godot_source\core\variant\binder_common.h:338)
[18] CallableCustomMethodPointer<EditorHelpSearch,void>::call (C:\godot_source\core\object\callable_method_pointer.h:107)
[19] Callable::callp (C:\godot_source\core\variant\callable.cpp:58)
[20] Object::emit_signalp (C:\godot_source\core\object\object.cpp:1288)
[21] Node::emit_signalp (C:\godot_source\scene\main\node.cpp:4315)
[22] Object::emit_signal<> (C:\godot_source\core\object\object.h:934)
[23] Tree::gui_input (C:\godot_source\scene\gui\tree.cpp:4173)
[24] Control::_call_gui_input (C:\godot_source\scene\gui\control.cpp:1866)
[25] Viewport::_gui_call_input (C:\godot_source\scene\main\viewport.cpp:1728)
[26] Viewport::_gui_input_event (C:\godot_source\scene\main\viewport.cpp:1959)
[27] Viewport::push_input (C:\godot_source\scene\main\viewport.cpp:3427)
[28] Window::_window_input (C:\godot_source\scene\main\window.cpp:1839)
[29] call_with_variant_args_helper<Window,Ref<InputEvent> const &,0> (C:\godot_source\core\variant\binder_common.h:223)
[30] call_with_variant_args<Window,Ref<InputEvent> const &> (C:\godot_source\core\variant\binder_common.h:338)
[31] CallableCustomMethodPointer<Window,void,Ref<InputEvent> const &>::call (C:\godot_source\core\object\callable_method_pointer.h:107)
[32] Callable::callp (C:\godot_source\core\variant\callable.cpp:58)
[33] Callable::call<Ref<InputEvent> > (C:\godot_source\core\variant\variant.h:953)
[34] DisplayServerWindows::_dispatch_input_event (C:\godot_source\platform\windows\display_server_windows.cpp:4414)
[35] DisplayServerWindows::_dispatch_input_events (C:\godot_source\platform\windows\display_server_windows.cpp:4385)
[36] Input::_parse_input_event_impl (C:\godot_source\core\input\input.cpp:903)
[37] Input::flush_buffered_events (C:\godot_source\core\input\input.cpp:1184)
[38] DisplayServerWindows::process_events (C:\godot_source\platform\windows\display_server_windows.cpp:3798)
[39] OS_Windows::run (C:\godot_source\platform\windows\os_windows.cpp:2278)
[40] widechar_main (C:\godot_source\platform\windows\godot_windows.cpp:97)
[42] main (C:\godot_source\platform\windows\godot_windows.cpp:136)
[43] WinMain (C:\godot_source\platform\windows\godot_windows.cpp:150)
[44] __scrt_common_main_seh (D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288)
[45] <couldn't map PC to fn name>
-- END OF C++ BACKTRACE --

get_class_icon() has a fallback argument and seems like it can be called with different fallbacks for the same icon, which means that it could incorrectly return an empty icon when there was fallback available. Some code expects the icon to never be null (as seen in the crash above).

To replicate the crash make a script that extends Node and open Node's doc page.

[akien-mga]

To replicate the crash make a script that extends Node and open Node's doc page.

I don't seem to reproduce the crash like this on Linux (with a dev build).

I'm trying to figure out how likely users are to run into this crash and whether to rebuild 4.5-dev5 (just finished building) with this fix.

[Macksaur]

I can be working for 30 mins or a few hours. But it will crash when I interact with the docs at some point, if I ctrl-click something to jump, or to read about it? Kaboom, it will crash. Cured after I merged this though.

[Repiteo]

Thanks!