Fix crash when pasting images in post composer (#1214)

The app would crash with 'attempt to create backwards NSTextRange from 1 to 0' when pasting images into a post. This was caused by the text system trying to query text ranges while they were being modified.

Fixed by:
- Storing the original selected range before text storage modifications
- Completing all text storage edits before updating the selection
- Deferring selection updates to the next run loop cycle to avoid conflicts with the text system's internal state
- Adding bounds checking to ensure valid cursor positions

This prevents race conditions between our selection updates and the text system's attribute queries.

Author: JonET

App/Composition/CompositionMenuTree.swift

@@ -219,18 +219,30 @@ final class CompositionMenuTree: NSObject {
 
         // Directly modify the textStorage instead of setting a whole new attributedText on the UITextView, which can be slow and jumps the text view around. We'll need to post our own text changed notification too.
         let storage = textView.textStorage
+        let originalSelectedRange = textView.selectedRange
         storage.beginEditing()
         storage.replaceCharacters(in: textView.selectedRange, with: string)
         textView.font = font
         textView.textColor = textColor
-        if
-            let selection = textView.selectedTextRange,
-            let afterImagePosition = textView.position(from: selection.end, offset: 1)
-        {
-            textView.selectedTextRange = textView.textRange(from: afterImagePosition, to: afterImagePosition)
-        }
         storage.endEditing()
 
+        // Calculate new cursor position after the inserted image
+        let newCursorLocation = originalSelectedRange.location + string.length
+        
+        // Defer the selection update to avoid conflicts with the text system
+        // This prevents the crash when the system tries to query text ranges during the update
+        DispatchQueue.main.async { [weak textView] in
+            guard let textView = textView else { return }
+            
+            // Ensure the new position is within valid bounds
+            if newCursorLocation <= textView.textStorage.length {
+                textView.selectedRange = NSRange(location: newCursorLocation, length: 0)
+            } else {
+                // If somehow we're beyond the text bounds, place cursor at the end
+                textView.selectedRange = NSRange(location: textView.textStorage.length, length: 0)
+            }
+        }
+        
         NotificationCenter.default.post(name: UITextView.textDidChangeNotification, object: textView)
     }