Fix crash when opening filter popover's expression editor (#35022)

Co-authored-by: Uladzimir Havenchyk <125459446+uladzimirdev@users.noreply.github.com>

e2e/test/scenarios/filters/reproductions/34794-filter-popover-navigation-error.cy.spec.js

+import {
+  filter,
+  getNotebookStep,
+  openOrdersTable,
+  popover,
+  restore,
+} from "e2e/support/helpers";
+
+describe("issue 34794", () => {
+  beforeEach(() => {
+    restore();
+    cy.signInAsNormalUser();
+  });
+
+  it("should not crash when navigating to filter popover's custom expression section (metabase#34794)", () => {
+    openOrdersTable({ mode: "notebook" });
+
+    filter({ mode: "notebook" });
+    popover().within(() => {
+      cy.findByText("Created At").click();
+      cy.icon("chevronleft").click(); // go back to the main filter popover
+      cy.findByText("Custom Expression").click();
+      cy.findByLabelText("Expression").type("[Total] > 10").blur();
+      cy.button("Done").click();
+    });
+
+    getNotebookStep("filter")
+      .findByText("Total is greater than 10")
+      .should("be.visible");
+  });
+});

frontend/src/metabase/query_builder/components/filters/FilterPopover/FilterPopover.tsx

@@ -165,10 +165,14 @@ export function FilterPopover({
   };
 
   if (editingFilter) {
+    const filterMBQL = filter?.raw();
+    const expression = isExpression(filterMBQL)
+      ? (filterMBQL as Expression)
+      : undefined;
     return (
       <ExpressionWidget
         query={query}
-        expression={filter?.raw() as Expression | undefined}
+        expression={expression}
         startRule="boolean"
         header={<ExpressionWidgetHeader onBack={handleExpressionWidgetClose} />}
         onChangeExpression={handleExpressionChange}