Fixed TextFlow children concurrency access (#1142)

Author: Jugen

richtextfx/src/main/java9/org/fxmisc/richtext/TextFlowLayout.java

@@ -75,7 +75,7 @@ int getLineCount() {
         double totLines = 0.0, prevMaxY = -1.0;
         int totCharSoFar = 0;
 
-        for ( Node n : flow.getChildren() ) if ( n.isManaged() ) {
+        for ( Node n : flow.getChildrenUnmodifiable() ) if ( n.isManaged() ) {
 
             Bounds nodeBounds = n.getBoundsInParent();
             int length = (n instanceof Text) ? ((Text) n).getText().length() : 1;