Adding check for null Screen which was causing a crash in our project. (#245)

Author: nhojb

src/textbox.cpp

@@ -513,6 +513,8 @@ bool TextBox::checkFormat(const std::string &input, const std::string &format) {
 bool TextBox::copySelection() {
     if (mSelectionPos > -1) {
         Screen *sc = dynamic_cast<Screen *>(this->window()->parent());
+        if (!sc)
+            return false;
 
         int begin = mCursorPos;
         int end = mSelectionPos;
@@ -530,6 +532,8 @@ bool TextBox::copySelection() {
 
 void TextBox::pasteFromClipboard() {
     Screen *sc = dynamic_cast<Screen *>(this->window()->parent());
+    if (!sc)
+        return;
     const char* cbstr = glfwGetClipboardString(sc->glfwWindow());
     if (cbstr)
         mValueTemp.insert(mCursorPos, std::string(cbstr));