[Autofill Assistant] Clear form on error

In case of an error (or generally stopping the flow), the usual
suspects were cleared. The form was not and stayed visible.
Upon interaction with the form in a shut-down state, the system
would crash.

This is not easily reproducible in normal use but may appear when
tab switching is enabled.

Bug: b/143753128
Change-Id: I3301f1299fe6a40fe3780752aa3b92b32becc538
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1893999Reviewed-by: Clemens Arbesser <arbesser@google.com>
Commit-Queue: Sandro Maggi <sandromaggi@google.com>
Cr-Commit-Position: refs/heads/master@{#711702}

components/autofill_assistant/browser/controller.cc

@@ -511,6 +511,7 @@ void Controller::EnterStoppedState() {
   SetDetails(nullptr);
   SetUserActions(nullptr);
   SetCollectUserDataOptions(nullptr, nullptr);
+  SetForm(nullptr, base::DoNothing());
   EnterState(AutofillAssistantState::STOPPED);
 }