Guard against null object when trying to fetch a function

src/browser/dom/event_target.zig

@@ -222,4 +222,10 @@ test "Browser.DOM.EventTarget" {
         .{ "content.dispatchEvent(new Event('he'));", null },
         .{ "obj1.calls", "1" },
     }, .{});
+
+    // doesn't crash on null receiver
+    try runner.testCases(&.{
+        .{ "content.addEventListener('he2', null);", null },
+        .{ "content.dispatchEvent(new Event('he2'));", null },
+    }, .{});
 }

src/runtime/js.zig

@@ -1400,7 +1400,11 @@ pub fn Env(comptime State: type, comptime WebApis: type) type {
             }
 
             pub fn getFunction(self: JsObject, name: []const u8) !?Function {
+                if (self.isNullOrUndefined()) {
+                    return null;
+                }
                 const scope = self.scope;
+
                 const js_name = v8.String.initUtf8(scope.isolate, name);
 
                 const js_value = try self.js_obj.getValue(scope.context, js_name.toName());
@@ -1409,6 +1413,10 @@ pub fn Env(comptime State: type, comptime WebApis: type) type {
                 }
                 return try scope.createFunction(js_value);
             }
+
+            pub fn isNullOrUndefined(self: JsObject) bool {
+                return self.js_obj.toValue().isNullOrUndefined();
+            }
         };
 
         // This only exists so that we know whether a function wants the opaque