fix(react): Don't crash ErrorBoundary when error.cause is not an Error

packages/react/src/error.ts

@@ -15,6 +15,10 @@ export function setCause(error: Error & { cause?: Error }, cause: Error): void {
       return;
     }
     if (error.cause) {
+      // We can only link to an `Error` cause, not e.g. a string (`new Error('x', { cause: 'y' })`).
+      if (!isError(error.cause)) {
+        return;
+      }
       seenErrors.add(error);
       return recurse(error.cause, cause);
     }

packages/react/test/errorboundary.test.tsx

@@ -489,6 +489,28 @@ describe('ErrorBoundary', () => {
       expect(cause.message).toEqual(thirdError.message);
     });
 
+    it('handles when `error.cause` is not an Error', () => {
+      function CustomBam(): JSX.Element {
+        const error = new Error('bam');
+        // @ts-expect-error Need to set cause on error
+        error.cause = 'timeout';
+        throw error;
+      }
+
+      const { container } = render(
+        <TestApp fallback={<p>You have hit an error</p>} errorComp={<CustomBam />}>
+          <h1>children</h1>
+        </TestApp>,
+      );
+
+      const btn = screen.getByTestId('errorBtn');
+      fireEvent.click(btn);
+
+      expect(container.innerHTML).toContain('You have hit an error');
+      expect(mockCaptureException).toHaveBeenCalledTimes(1);
+      expect(mockCaptureException.mock.calls[0]?.[0].cause).toBe('timeout');
+    });
+
     it('handles when `error.cause` is recursive', () => {
       const mockOnError = vi.fn();