Fix crash when nativeSymbol index is out of bounds in assembly view

src/reducers/url-state.ts

@@ -606,12 +606,21 @@ const assemblyView: Reducer<AssemblyViewState> = (
       const { nativeSymbols, currentNativeSymbol, shouldOpenAssemblyView } =
         action;
       const shouldScroll = action.scrollToInstructionAddress !== undefined;
+
+      // Clamp currentNativeSymbol to null if it is out of bounds, so that
+      // "currentNativeSymbol is always in-bounds or null" holds.
+      const clampedNativeSymbol =
+        currentNativeSymbol !== null &&
+        currentNativeSymbol < nativeSymbols.length
+          ? currentNativeSymbol
+          : null;
+
       return {
         scrollGeneration: shouldScroll
           ? state.scrollGeneration + 1
           : state.scrollGeneration,
         nativeSymbols,
-        currentNativeSymbol,
+        currentNativeSymbol: clampedNativeSymbol,
         isOpen: state.isOpen || shouldOpenAssemblyView,
         scrollToInstructionAddress: action.scrollToInstructionAddress,
         highlightedInstruction: action.highlightedInstructionAddress,

src/test/store/bottom-box.test.ts

@@ -345,6 +345,29 @@ describe('bottom box', function () {
     ).toBeOneOf([0x40, 0x45]);
   });
 
+  it('returns null for getAssemblyViewNativeSymbol when nativeSymbols is empty but initialNativeSymbol is 0', function () {
+    // initialNativeSymbol = 0, even when nativeSymbols is empty
+    // (e.g., when the frame has no native symbol).
+    // The selector must return null (not undefined)
+    // to avoid a crash when downstream code accesses .libIndex on it.
+    const { dispatch, getState } = setup();
+
+    dispatch(
+      updateBottomBoxContentsAndMaybeOpen('calltree', {
+        libIndex: null,
+        sourceIndex: null,
+        nativeSymbols: [],
+        initialNativeSymbol: 0,
+        highlightedLineNumber: null,
+        highlightedInstructionAddress: null,
+      })
+    );
+
+    expect(
+      UrlStateSelectors.getAssemblyViewNativeSymbol(getState())
+    ).toBeNull();
+  });
+
   // Further ideas for tests:
   //
   // - A test with multiple threads: Open the assembly view for a symbol, switch