Nested slots with default value and store: Cannot read property 'value' of undefined

[iljabauer]

Describe the bug
There is an Outer Component with a slot, which defaults to Inner component. Inner component consisting of an input field bound to a store writable object.
If we now pass something to the slot, e.g the Inner component surrounded by a div, input binding will break and log an "Cannot read property 'value' of undefined" error.

Logs

VM210:422 Uncaught (in promise) TypeError: Cannot read property 'value' of undefined
    at Object.update [as p] (eval at handle_message (VM204 about:srcdoc:13), <anonymous>:422:37)
    at update (eval at handle_message (VM204 about:srcdoc:13), <anonymous>:159:25)
    at flush (eval at handle_message (VM204 about:srcdoc:13), <anonymous>:133:17)
update @ VM210:422
update @ VM210:159
flush @ VM210:133
Promise.then (async)
schedule_update @ VM210:119
make_dirty @ VM210:219
eval @ VM210:253
eval @ VM210:444
set @ VM210:364
update @ VM210:372
input_input_handler @ VM210:447

To Reproduce
REPL 1 with bug: https://svelte.dev/repl/48555174f1a34bbe8268404c7a7f0b99?version=3.12.1
REPL 2 working without slot default: https://svelte.dev/repl/31a272fac83047c282a8fe721751a884?version=3.12.1

Expected behavior
It should work like REPL 2

Information about your Svelte project:

• Your browser and the version: Chromium: 77.0.3865.120

• Your operating system: OS X 10.15

• Svelte version: 3.12.1

Severity
The bug is not urgent, as I can work around it by removing the slot default like in REPL 2. But its quite inconvenient

[Rich-Harris]

The problem here is that the Inner component gets created whether or not there's a default slot:

function create_fragment(ctx) {
	var current;

	const default_slot_template = ctx.$$slots.default;
	const default_slot = create_slot(default_slot_template, ctx, null);

	var inner = new Inner({});

	return {...};
}

It then tries to update itself despite not having been 'created'. We need to ensure that components in slot fallback content are only created when appropriate.

[Conduitry]

This has been fixed in 3.20.0 - https://svelte.dev/repl/48555174f1a34bbe8268404c7a7f0b99?version=3.20.0