Fix getSelectedBlockClientId selector (#12214)

* Fix getSelectedBlockClientId selector

At the moment, we don't have a way to clear or update
block selection for UNDO / REDO actions.

By inverting the dependencies between getSelectedBlockClientId
and getSelectedBlock we make sure we always return
a valid clientId in getSelectedBlockClientId.

We still have to fix the UNDO/REDO problem internally,
but, in the meantime, this patch fixes the API.

* Fix tests

* Revert "Fix getSelectedBlockClientId selector"

This reverts commit cfa3066.

* Revert "Fix tests"

This reverts commit 60abeab.

* Use a more performant variant

packages/editor/src/store/selectors.js

@@ -853,7 +853,10 @@ export function hasSelectedBlock( state ) {
  */
 export function getSelectedBlockClientId( state ) {
 	const { start, end } = state.blockSelection;
-	return start === end && start ? start : null;
+	// We need to check the block exists because the current state.blockSelection reducer
+	// doesn't take into account the UNDO / REDO actions to update selection.
+	// To be removed when that's fixed.
+	return start && start === end && !! state.editor.present.blocks.byClientId[ start ] ? start : null;
 }
 
 /**

packages/editor/src/store/test/selectors.js

@@ -2571,6 +2571,7 @@ describe( 'selectors', () => {
 
 		it( 'should return the selected block ClientId', () => {
 			const state = {
+				editor: { present: { blocks: { byClientId: { 23: { name: 'fake block' } } } } },
 				blockSelection: { start: 23, end: 23 },
 			};