Fix null access crash

Author: bdero

impeller/compiler/switches.cc

@@ -297,7 +297,8 @@ std::vector<TargetPlatform> Switches::PlatformsToCompile() const {
 }
 
 TargetPlatform Switches::SelectDefaultTargetPlatform() const {
-  if (target_platform_ == TargetPlatform::kUnknown) {
+  if (target_platform_ == TargetPlatform::kUnknown &&
+      !runtime_stages_.empty()) {
     return runtime_stages_.front();
   }
   return target_platform_;

impeller/compiler/switches.h

@@ -67,7 +67,7 @@ class Switches {
   // Use |SelectDefaultTargetPlatform|.
   TargetPlatform target_platform_ = TargetPlatform::kUnknown;
   // Use |PlatformsToCompile|.
-  std::vector<TargetPlatform> runtime_stages_ = {};
+  std::vector<TargetPlatform> runtime_stages_;
 };
 
 }  // namespace compiler

impeller/compiler/types.h

@@ -8,6 +8,7 @@
 #include <codecvt>
 #include <locale>
 #include <map>
+#include <optional>
 #include <string>
 
 #include "flutter/fml/macros.h"