Make memory watching work before sim is started

src/emulator.rs

@@ -518,11 +518,12 @@ impl Emulator {
                 let Some(region) = region.upgrade() else {
                     return false;
                 };
-                let Some(sim) = self.sims.get_mut(range.sim.to_index()) else {
-                    return false;
-                };
                 self.buffer.clear();
+                if let Some(sim) = self.sims.get_mut(range.sim.to_index()) {
                     sim.read_memory(range.start, range.length, &mut self.buffer);
+                } else {
+                    self.buffer.extend(std::iter::repeat_n(0, range.length));
+                }
                 region.update(&self.buffer);
                 true
             });

src/graphics.rs

@@ -142,7 +142,6 @@ impl BufferPool {
     }
 }
 
-#[derive(Debug)]
 struct Buffer {
     data: Mutex<Vec<u8>>,
     id: AtomicU64,
@@ -155,3 +154,10 @@ impl Buffer {
         }
     }
 }
+impl std::fmt::Debug for Buffer {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("Buffer")
+            .field("id", &self.id)
+            .finish_non_exhaustive()
+    }
+}