Catch NPE error and use a default value for a Pushbutton

DEVSIX-2526

Author: Samuel Huylebroeck

forms/src/main/java/com/itextpdf/forms/fields/PdfFormField.java

@@ -3222,6 +3222,9 @@ protected void drawButton(PdfCanvas canvas, float x, float y, float width, float
         if (color == null) {
             color = ColorConstants.BLACK;
         }
+        if(text == null){
+            text="";
+        }
 
         Paragraph paragraph = new Paragraph(text).setFont(font).setFontSize(fontSize).setMargin(0).setMultipliedLeading(1).
                 setVerticalAlignment(VerticalAlignment.MIDDLE);

forms/src/test/java/com/itextpdf/forms/FormFieldFlatteningTest.java

@@ -230,4 +230,10 @@ public void flattenReadOnly() throws IOException{
         pdfDoc.close();
         Assert.assertFalse(isReadOnly);
     }
+
+    @Test
+    public void fieldsRegeneratePushButtonWithoutCaption() throws IOException, InterruptedException {
+        fillTextFieldsThenFlattenThenCompare("pushbutton_without_caption");
+    }
+
 }