LibWeb: Avoid crashing because of empty incumbent settings object stack

This fixes an issue where we end up in a state where we have no
execution context + a main thread event loop with an empty incumbent
settings object stack.

Userland/Libraries/LibWeb/FileAPI/Blob.cpp

@@ -282,7 +282,19 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<Streams::ReadableStream>> Blob::get_stream(
 
             // 2. Queue a global task on the file reading task source given blob’s relevant global object to perform the following steps:
             HTML::queue_global_task(HTML::Task::Source::FileReading, realm.global_object(), [stream, bytes = move(bytes)]() {
-                HTML::TemporaryExecutionContext execution_context { Bindings::host_defined_environment_settings_object(stream->realm()) };
+                // NOTE: Using an TemporaryExecutionContext here results in a crash in the method HTML::incumbent_settings_object()
+                //       since we end up in a state where we have no execution context + an event loop with an empty incumbent
+                //       settings object stack. We still need an execution context therefore we push the realm's execution context
+                //       onto the realm's VM, and we need an incumbent settings object which is pushed onto the incumbent settings
+                //       object stack by EnvironmentSettings::prepare_to_run_callback().
+                auto& realm = stream->realm();
+                auto& environment_settings = Bindings::host_defined_environment_settings_object(realm);
+                realm.vm().push_execution_context(environment_settings.realm_execution_context());
+                environment_settings.prepare_to_run_callback();
+                ScopeGuard const guard = [&environment_settings, &realm] {
+                    environment_settings.clean_up_after_running_callback();
+                    realm.vm().pop_execution_context();
+                };
 
                 // 1. If bytes is failure, then error stream with a failure reason and abort these steps.
                 // 2. Let chunk be a new Uint8Array wrapping an ArrayBuffer containing bytes. If creating the ArrayBuffer throws an exception, then error stream with that exception and abort these steps.