ignore errors when sending FrameTree.initRequest message

lib/frame-tree/bg/frame-tree.js

@@ -41,7 +41,8 @@ this.FrameTree = (() => {
 					}
 				};
 				browser.runtime.onMessage.addListener(onMessage);
-				browser.tabs.sendMessage(tabId, { method: "FrameTree.initRequest", windowId: "0", index: 0 });
+				browser.tabs.sendMessage(tabId, { method: "FrameTree.initRequest", windowId: "0", index: 0 })
+					.catch(() => {/* ignored */ });
 			});
 		}
 	};