Fix error when trying to grab selection of a void node while read only (#3894)

* fix(react-editor): text node spacer can be non-existant if editor is in readonly mode

* v0.59.1

* Revert "v0.59.1"

This reverts commit 58cb7f1d61e81788e0257bace9be59bc47be8851.

* Update react-editor.ts

* Update react-editor.ts

* Create rude-lemons-crash.md

Co-authored-by: Ian Storm Taylor <ian@ianstormtaylor.com>

.changeset/rude-lemons-crash.md

@@ -0,0 +1,5 @@
+---
+'slate-react': patch
+---
+
+Fixed an error that happened when selecting void nodes in a read-only editor.

packages/slate-react/src/plugin/react-editor.ts

@@ -478,14 +478,19 @@ export const ReactEditor = {
       } else if (voidNode) {
         // For void nodes, the element with the offset key will be a cousin, not an
         // ancestor, so find it by going down from the nearest void parent.
-
         leafNode = voidNode.querySelector('[data-slate-leaf]')!
-        textNode = leafNode.closest('[data-slate-node="text"]')!
-        domNode = leafNode
-        offset = domNode.textContent!.length
-        domNode.querySelectorAll('[data-slate-zero-width]').forEach(el => {
-          offset -= el.textContent!.length
-        })
+
+        // COMPAT: In read-only editors the leaf is not rendered.
+        if (!leafNode) {
+          offset = 1
+        } else {
+          textNode = leafNode.closest('[data-slate-node="text"]')!
+          domNode = leafNode
+          offset = domNode.textContent!.length
+          domNode.querySelectorAll('[data-slate-zero-width]').forEach(el => {
+            offset -= el.textContent!.length
+          })
+        }
       }
 
       // COMPAT: If the parent node is a Slate zero-width space, editor is