ui: Incorrect null-check operator in `_LoadingPlaceholderPageState.build`

[chrisbobbe]

To reproduce:

1. Use a debug build so you can see errors printed to the console
2. Log into an account
3. Log out of the account ("Choose account" > three-dots menu > "Log out")
4. See the following in the console:

======== Exception caught by widgets library =======================================================
The following _TypeError was thrown building _LoadingPlaceholderPage(dirty, dependencies: [_GlobalStoreInheritedWidget, _LocalizationsScope-[GlobalKey#c7674]], state: _LoadingPlaceholderPageState#4549b):
Null check operator used on a null value

The relevant error-causing widget was: 
  _LoadingPlaceholderPage _LoadingPlaceholderPage:file:///Users/chrisbobbe/dev/zulip-flutter/lib/widgets/home.dart:36:31
When the exception was thrown, this was the stack: 
#0      _LoadingPlaceholderPageState.build (package:zulip/widgets/home.dart:184:36)
#1      StatefulElement.build (package:flutter/src/widgets/framework.dart:5841:27)
#2      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5733:15)
#3      StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5892:11)
#4      Element.rebuild (package:flutter/src/widgets/framework.dart:5445:7)
#5      ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:5715:5)
#6      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:5883:11)
#7      ComponentElement.mount (package:flutter/src/widgets/framework.dart:5709:5)

This is a latent bug introduced in a97ac42 (fyi @PIG208). This widget shouldn't assume that an account for its accountId param exists; in particular, it won't exist in the short period after the account is removed from the database (for logout) and before routeToRemoveOnLogout is processed in a post-frame callback.

I see two plausible fixes:

• Remove the accountId param and pass…hmm, I guess a realmUrl param, for the "try another account" message. Or:
• Keep the accountId param but give it dartdoc with a reminder that the corresponding account might not exist, saying why.

In either case we should add a test that would fail before the fix but passes after it.

[chrisbobbe]

Tentatively assigning @PIG208 as the author of a97ac42.