[mod] Fix all issues relate to long click in Android version 7.0 (#657, #527)

Author: tranductam2802

android/src/main/java/com/pichillilorenzo/flutter_inappwebview/InAppWebView/InAppWebView.java

@@ -1966,7 +1966,7 @@ public void adjustFloatingContextMenuPosition() {
       @Override
       public void onReceiveValue(String value) {
         if (floatingContextMenu != null) {
-          if (value != null && !value.equals("null")) {
+          if (value != null && !value.equalsIgnoreCase("null")) {
             int x = contextMenuPoint.x;
             int y = (int) ((Float.parseFloat(value) * scale) + (floatingContextMenu.getHeight() / 3.5));
             contextMenuPoint.y = y;
@@ -1986,7 +1986,7 @@ public void getSelectedText(final ValueCallback<String> resultCallback) {
     evaluateJavascript(getSelectedTextJS, new ValueCallback<String>() {
       @Override
       public void onReceiveValue(String value) {
-        value = (value != null) ? value.substring(1, value.length() - 1) : null;
+        value = (value != null && !value.equalsIgnoreCase("null")) ? value.substring(1, value.length() - 1) : null;
         resultCallback.onReceiveValue(value);
       }
     });