Don't show warnings when removing handlers. (#18633)

These are removed in destructors.

shell/platform/linux/fl_basic_message_channel.cc

@@ -182,8 +182,10 @@ G_MODULE_EXPORT void fl_basic_message_channel_set_message_handler(
 
   // Don't set handler if channel closed
   if (self->channel_closed) {
-    g_warning(
-        "Attempted to set message handler on closed FlBasicMessageChannel");
+    if (handler != nullptr) {
+      g_warning(
+          "Attempted to set message handler on a closed FlBasicMessageChannel");
+    }
     if (destroy_notify != nullptr)
       destroy_notify(user_data);
     return;

shell/platform/linux/fl_binary_messenger.cc

@@ -175,9 +175,11 @@ G_MODULE_EXPORT void fl_binary_messenger_set_message_handler_on_channel(
 
   // Don't set handlers if engine already gone
   if (self->engine == nullptr) {
-    g_warning(
-        "Attempted to set message handler on closed FlBinaryMessenger without "
-        "engine");
+    if (handler != nullptr) {
+      g_warning(
+          "Attempted to set message handler on an FlBinaryMessenger without an "
+          "engine");
+    }
     if (destroy_notify != nullptr)
       destroy_notify(user_data);
     return;

shell/platform/linux/fl_method_channel.cc

@@ -140,7 +140,10 @@ G_MODULE_EXPORT void fl_method_channel_set_method_call_handler(
 
   // Don't set handler if channel closed
   if (self->channel_closed) {
-    g_warning("Attempted to set method call handler on closed FlMethodChannel");
+    if (handler != nullptr) {
+      g_warning(
+          "Attempted to set method call handler on a closed FlMethodChannel");
+    }
     if (destroy_notify != nullptr)
       destroy_notify(user_data);
     return;