PY-12932 Fix trailing comment detection when it's the first child element

python/src/com/jetbrains/python/formatter/PyPreFormatProcessor.java

@@ -132,6 +132,6 @@ public class PyPreFormatProcessor implements PreFormatProcessor {
 
   private static boolean isTrailingComment(@NotNull PsiComment comment) {
     final PsiElement prevElement = comment.getPrevSibling();
-    return !(prevElement instanceof PsiWhiteSpace) || !prevElement.textContains('\n');
+    return prevElement != null && (!(prevElement instanceof PsiWhiteSpace) || !prevElement.textContains('\n'));
   }
 }

python/testData/formatter/commentedCodeFragmentIgnored.py

@@ -1,3 +1,4 @@
+#    This normal comment won't be touched as well
 # for a in [1, 2, 3]:
 #     if a == 1:
 #         print 'x'

python/testData/formatter/commentedCodeFragmentIgnored_after.py

@@ -1,3 +1,4 @@
+#    This normal comment won't be touched as well
 # for a in [1, 2, 3]:
 #     if a == 1:
 #         print 'x'