fix: prevent crash when dragging blocks after rapidly creating projects

js/__tests__/block-drag-controller.test.js

@@ -895,6 +895,23 @@ describe("BlockDragController", () => {
 
             debugSpy.mockRestore();
         });
+        it("guards against a connection pointing at a not-yet-populated slot in blockList", () => {
+            // blockList is pre-sized for the whole incoming batch (as blocks.js's
+            // chunked loader does via `_loadCounter = blockObjs.length`), but a
+            // later chunk hasn't been written into it yet, so index 1 is a genuine
+            // array hole (undefined) rather than an explicit null placeholder.
+            // Regression test for the "Cannot read properties of undefined
+            // (reading 'connections')" crash on rapid next-project + play.
+            const blockList = [];
+            blockList[0] = { connections: [null, 1] };
+            blockList.length = 3;
+            const blocks = makeBlocks(blockList);
+            const debugSpy = jest.spyOn(console, "debug").mockImplementation(() => {});
+
+            expect(() => blocks.findDragGroup(0)).not.toThrow();
+
+            debugSpy.mockRestore();
+        });
 
         it("_calculateDragGroup stops recursing once the loop counter exceeds the block list length", () => {
             const blockList = [

js/block-drag-controller.js

@@ -124,9 +124,12 @@ class BlockDragController {
         }
 
         const myBlock = blocks.blockList[blk];
-        /** If this happens, something is really broken. */
-        if (myBlock === null) {
-            console.debug("null block encountered... this is bad. " + blk);
+        /** If this happens, something is really broken.
+         *  Covers both an explicit null entry and an undefined one (e.g. an
+         *  index that points past the currently-loaded portion of
+         *  blockList, or a block removed from the array). */
+        if (!myBlock) {
+            console.debug("null/missing block encountered... this is bad. " + blk);
             return;
         }