Raise a helpful error if you access context before rendering (#863)

If you access the `context` before rendering, the error message says
`nil` doesn’t respond to `user_context`. This PR makes `context` return
`nil` instead. Opening as a draft as I want to write a test for this
before merging.

lib/phlex/sgml.rb

@@ -94,7 +94,19 @@ def internal_call(parent: nil, state: nil, &block)
 	end
 
 	def context
-		@_state.user_context
+		if rendering?
+			@_state.user_context
+		else
+			raise Phlex::ArgumentError.new(<<~MESSAGE)
+				You can’t access the context before the component has started rendering.
+			MESSAGE
+		end
+	end
+
+	# Returns `false` before rendering and `true` once the component has started rendering.
+	# It will not reset back to false after rendering.
+	def rendering?
+		!!@_state
 	end
 
 	# Output plain text.

quickdraw/context.test.rb

@@ -27,3 +27,13 @@
 
 	assert_equal_html b.new.call, %(<h1>Hello, World!</h1>)
 end
+
+test "raises an ArgumentError if you access the context before rendering" do
+	component = Phlex::HTML.new
+
+	error = assert_raises(Phlex::ArgumentError) { component.context }
+
+	assert_equal error.message, <<~MESSAGE
+		You can’t access the context before the component has started rendering.
+	MESSAGE
+end