Fix crash truncating a string of a node with a zero size

The crash happens in the placeholderImage of ASTextNode. The node is not visible in the time it try to get the `placeholderImage` and so the `visibleRange` has count 0 and a crash happens while accessing the first object of an empty array.

AsyncDisplayKit/TextKit/ASTextKitRenderer+TextChecking.mm

@@ -59,7 +59,6 @@
   NSAttributedString *truncationAttributedString = self.attributes.truncationAttributedString;
 
   // get the index of the last character, so we can handle text in the truncation token
-  NSRange visibleRange = self.truncater.visibleRanges[0];
   __block NSRange truncationTokenRange = { NSNotFound, 0 };
 
   [truncationAttributedString enumerateAttribute:ASTextKitTruncationAttributeName inRange:NSMakeRange(0, truncationAttributedString.length)
@@ -75,6 +74,7 @@
     truncationTokenRange = { 0, truncationAttributedString.length };
   }
 
+  NSRange visibleRange = self.truncater.firstVisibleRange;
   truncationTokenRange.location += NSMaxRange(visibleRange);
   
   __block CGFloat minDistance = CGFLOAT_MAX;

AsyncDisplayKit/TextKit/ASTextKitRenderer.h

@@ -78,7 +78,7 @@
  The character range from the original attributedString that is displayed by the renderer given the parameters in the
  initializer.
  */
-- (std::vector<NSRange>)visibleRanges;
+@property (nonatomic, assign, readonly) std::vector<NSRange> visibleRanges;
 
 /**
  The number of lines shown in the string.
@@ -86,3 +86,13 @@
 - (NSUInteger)lineCount;
 
 @end
+
+@interface ASTextKitRenderer (ASTextKitRendererConvenience)
+
+/**
+ Returns the first visible range or an NSRange with location of NSNotFound and size of 0 if no first visible
+ range exists
+ */
+@property (nonatomic, assign, readonly) NSRange firstVisibleRange;
+
+@end

AsyncDisplayKit/TextKit/ASTextKitRenderer.mm

@@ -262,3 +262,17 @@ static NSCharacterSet *_defaultAvoidTruncationCharacterSet()
 }
 
 @end
+
+@implementation ASTextKitRenderer (ASTextKitRendererConvenience)
+
+- (NSRange)firstVisibleRange
+{
+  std::vector<NSRange> visibleRanges = self.visibleRanges;
+  if (visibleRanges.size() > 0) {
+    return visibleRanges[0];
+  }
+  
+  return NSMakeRange(NSNotFound, 0);
+}
+
+@end

AsyncDisplayKit/TextKit/ASTextKitTailTruncater.mm

@@ -20,7 +20,6 @@
   NSCharacterSet *_avoidTailTruncationSet;
 }
 @synthesize visibleRanges = _visibleRanges;
-@synthesize truncationStringRect = _truncationStringRect;
 
 - (instancetype)initWithContext:(ASTextKitContext *)context
      truncationAttributedString:(NSAttributedString *)truncationAttributedString
@@ -185,4 +184,14 @@
   }];
 }
 
+- (NSRange)firstVisibleRange
+{
+  std::vector<NSRange> visibleRanges = _visibleRanges;
+  if (visibleRanges.size() > 0) {
+    return visibleRanges[0];
+  }
+
+  return NSMakeRange(NSNotFound, 0);
+}
+
 @end

AsyncDisplayKit/TextKit/ASTextKitTruncating.h

@@ -14,10 +14,21 @@
 
 #import "ASTextKitRenderer.h"
 
+NS_ASSUME_NONNULL_BEGIN
+
 @protocol ASTextKitTruncating <NSObject>
 
+/**
+ The character range from the original attributedString that is displayed by the renderer given the parameters in the
+ initializer.
+ */
 @property (nonatomic, assign, readonly) std::vector<NSRange> visibleRanges;
-@property (nonatomic, assign, readonly) CGRect truncationStringRect;
+
+/**
+ Returns the first visible range or an NSRange with location of NSNotFound and size of 0 if no first visible
+ range exists
+ */
+@property (nonatomic, assign, readonly) NSRange firstVisibleRange;
 
 /**
  A truncater object is initialized with the full state of the text.  It is a Single Responsibility Object that is
@@ -30,12 +41,14 @@
  The truncater should not store a strong reference to the context to prevent retain cycles.
  */
 - (instancetype)initWithContext:(ASTextKitContext *)context
-     truncationAttributedString:(NSAttributedString *)truncationAttributedString
-         avoidTailTruncationSet:(NSCharacterSet *)avoidTailTruncationSet;
+     truncationAttributedString:(NSAttributedString * _Nullable)truncationAttributedString
+         avoidTailTruncationSet:(NSCharacterSet * _Nullable)avoidTailTruncationSet;
 
 /**
- *  Actually do the truncation.
+ Actually do the truncation.
  */
 - (void)truncate;
 
 @end
+
+NS_ASSUME_NONNULL_END