code_block: Fix null-check ! errors during hot reloads

On hot-reload, `lerp` is triggered even when the theme has not
changed. Discussion:
  https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/Exceptions.20during.20hot.20reloads/near/1846095

I considered just going ahead with placing the web app's current
light-theme styles for code blocks (#754), but pretty soon I
realized there would still be some nulls in there after doing so,
and those would need to be treated like we're treating the nulls
here.

Author: chrisbobbe

lib/widgets/code_block.dart

@@ -842,23 +842,23 @@ class CodeBlockTextStyles {
       hll: TextStyle.lerp(a._hll, b._hll, t)!,
       c: TextStyle.lerp(a._c, b._c, t)!,
       err: TextStyle.lerp(a._err, b._err, t)!,
-      esc: TextStyle.lerp(a._esc, b._esc, t)!,
-      g: TextStyle.lerp(a._g, b._g, t)!,
+      esc: TextStyle.lerp(a._esc, b._esc, t),
+      g: TextStyle.lerp(a._g, b._g, t),
       k: TextStyle.lerp(a._k, b._k, t)!,
-      l: TextStyle.lerp(a._l, b._l, t)!,
-      n: TextStyle.lerp(a._n, b._n, t)!,
+      l: TextStyle.lerp(a._l, b._l, t),
+      n: TextStyle.lerp(a._n, b._n, t),
       o: TextStyle.lerp(a._o, b._o, t)!,
-      x: TextStyle.lerp(a._x, b._x, t)!,
-      p: TextStyle.lerp(a._p, b._p, t)!,
-      ch: TextStyle.lerp(a._ch, b._ch, t)!,
+      x: TextStyle.lerp(a._x, b._x, t),
+      p: TextStyle.lerp(a._p, b._p, t),
+      ch: TextStyle.lerp(a._ch, b._ch, t),
       cm: TextStyle.lerp(a._cm, b._cm, t)!,
       cp: TextStyle.lerp(a._cp, b._cp, t)!,
-      cpf: TextStyle.lerp(a._cpf, b._cpf, t)!,
+      cpf: TextStyle.lerp(a._cpf, b._cpf, t),
       c1: TextStyle.lerp(a._c1, b._c1, t)!,
       cs: TextStyle.lerp(a._cs, b._cs, t)!,
       gd: TextStyle.lerp(a._gd, b._gd, t)!,
       ge: TextStyle.lerp(a._ge, b._ge, t)!,
-      ges: TextStyle.lerp(a._ges, b._ges, t)!,
+      ges: TextStyle.lerp(a._ges, b._ges, t),
       gr: TextStyle.lerp(a._gr, b._gr, t)!,
       gh: TextStyle.lerp(a._gh, b._gh, t)!,
       gi: TextStyle.lerp(a._gi, b._gi, t)!,
@@ -873,7 +873,7 @@ class CodeBlockTextStyles {
       kp: TextStyle.lerp(a._kp, b._kp, t)!,
       kr: TextStyle.lerp(a._kr, b._kr, t)!,
       kt: TextStyle.lerp(a._kt, b._kt, t)!,
-      ld: TextStyle.lerp(a._ld, b._ld, t)!,
+      ld: TextStyle.lerp(a._ld, b._ld, t),
       m: TextStyle.lerp(a._m, b._m, t)!,
       s: TextStyle.lerp(a._s, b._s, t)!,
       na: TextStyle.lerp(a._na, b._na, t)!,
@@ -889,19 +889,19 @@ class CodeBlockTextStyles {
       nt: TextStyle.lerp(a._nt, b._nt, t)!,
       nv: TextStyle.lerp(a._nv, b._nv, t)!,
       nx: TextStyle.lerp(a._nx, b._nx, t)!,
-      py: TextStyle.lerp(a._py, b._py, t)!,
+      py: TextStyle.lerp(a._py, b._py, t),
       ow: TextStyle.lerp(a._ow, b._ow, t)!,
-      pm: TextStyle.lerp(a._pm, b._pm, t)!,
+      pm: TextStyle.lerp(a._pm, b._pm, t),
       w: TextStyle.lerp(a._w, b._w, t)!,
-      mb: TextStyle.lerp(a._mb, b._mb, t)!,
+      mb: TextStyle.lerp(a._mb, b._mb, t),
       mf: TextStyle.lerp(a._mf, b._mf, t)!,
       mh: TextStyle.lerp(a._mh, b._mh, t)!,
       mi: TextStyle.lerp(a._mi, b._mi, t)!,
       mo: TextStyle.lerp(a._mo, b._mo, t)!,
-      sa: TextStyle.lerp(a._sa, b._sa, t)!,
+      sa: TextStyle.lerp(a._sa, b._sa, t),
       sb: TextStyle.lerp(a._sb, b._sb, t)!,
       sc: TextStyle.lerp(a._sc, b._sc, t)!,
-      dl: TextStyle.lerp(a._dl, b._dl, t)!,
+      dl: TextStyle.lerp(a._dl, b._dl, t),
       sd: TextStyle.lerp(a._sd, b._sd, t)!,
       s2: TextStyle.lerp(a._s2, b._s2, t)!,
       se: TextStyle.lerp(a._se, b._se, t)!,
@@ -912,11 +912,11 @@ class CodeBlockTextStyles {
       s1: TextStyle.lerp(a._s1, b._s1, t)!,
       ss: TextStyle.lerp(a._ss, b._ss, t)!,
       bp: TextStyle.lerp(a._bp, b._bp, t)!,
-      fm: TextStyle.lerp(a._fm, b._fm, t)!,
+      fm: TextStyle.lerp(a._fm, b._fm, t),
       vc: TextStyle.lerp(a._vc, b._vc, t)!,
       vg: TextStyle.lerp(a._vg, b._vg, t)!,
       vi: TextStyle.lerp(a._vi, b._vi, t)!,
-      vm: TextStyle.lerp(a._vm, b._vm, t)!,
+      vm: TextStyle.lerp(a._vm, b._vm, t),
       il: TextStyle.lerp(a._il, b._il, t)!,
     );
   }

test/widgets/code_block_test.dart

@@ -0,0 +1,55 @@
+import 'package:checks/checks.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:zulip/widgets/app.dart';
+import 'package:zulip/widgets/code_block.dart';
+import 'package:zulip/widgets/page.dart';
+
+import '../model/binding.dart';
+
+void main() {
+  TestZulipBinding.ensureInitialized();
+
+  group('CodeBlockTextStyles', () {
+    group('lerp', () {
+      Future<BuildContext> contextWithZulipTheme(WidgetTester tester) async {
+        addTearDown(testBinding.reset);
+        await tester.pumpWidget(const ZulipApp());
+        await tester.pump();
+        final navigator = await ZulipApp.navigator;
+        navigator.push(MaterialWidgetRoute(page: Builder(
+          builder: (context) => const Placeholder())));
+        await tester.pumpAndSettle();
+        return tester.element(find.byType(Placeholder));
+      }
+
+      testWidgets('light -> light', (tester) async {
+        final context = await contextWithZulipTheme(tester);
+        final a = CodeBlockTextStyles.light(context);
+        final b = CodeBlockTextStyles.light(context);
+        check(() => CodeBlockTextStyles.lerp(a, b, 0.5)).returnsNormally();
+      });
+
+      testWidgets('light -> dark', (tester) async {
+        final context = await contextWithZulipTheme(tester);
+        final a = CodeBlockTextStyles.light(context);
+        final b = CodeBlockTextStyles.dark(context);
+        check(() => CodeBlockTextStyles.lerp(a, b, 0.5)).returnsNormally();
+      });
+
+      testWidgets('dark -> light', (tester) async {
+        final context = await contextWithZulipTheme(tester);
+        final a = CodeBlockTextStyles.dark(context);
+        final b = CodeBlockTextStyles.light(context);
+        check(() => CodeBlockTextStyles.lerp(a, b, 0.5)).returnsNormally();
+      });
+
+      testWidgets('dark -> dark', (tester) async {
+        final context = await contextWithZulipTheme(tester);
+        final a = CodeBlockTextStyles.dark(context);
+        final b = CodeBlockTextStyles.dark(context);
+        check(() => CodeBlockTextStyles.lerp(a, b, 0.5)).returnsNormally();
+      });
+    });
+  });
+}