[engine] null check texture registry in OnPlatformViewMarkTextureFrameAvailable. (#53334)

From the issue, it looks like an NPE in this closure, we're not sure what else it could be besides the weak ptr - which could be null.

Fixes flutter/flutter#149895

Author: jonahwilliams

shell/common/shell.cc

@@ -1186,6 +1186,9 @@ void Shell::OnPlatformViewMarkTextureFrameAvailable(int64_t texture_id) {
   // Tell the rasterizer that one of its textures has a new frame available.
   task_runners_.GetRasterTaskRunner()->PostTask(
       [rasterizer = rasterizer_->GetWeakPtr(), texture_id]() {
+        if (!rasterizer) {
+          return;
+        }
         auto registry = rasterizer->GetTextureRegistry();
 
         if (!registry) {