Dont give nullptr as response to requestStateFile(); Fix build

Signed-off-by: falkTX <falktx@falktx.com>

distrho/src/DistrhoUIPrivateData.hpp

@@ -337,9 +337,9 @@ inline void PluginWindow::onFileSelected(const char* const filename)
     {
         ui->uiData->uiStateFileKeyRequest = nullptr;
         // notify DSP
-        ui->setState(key, filename);
+        ui->setState(key, filename != nullptr ? filename : "");
         // notify UI
-        ui->stateChanged(key, filename);
+        ui->stateChanged(key, filename != nullptr ? filename : "");
         std::free(key);
         return;
     }

examples/FileHandling/NanoButton.hpp

@@ -22,6 +22,7 @@
 #define NANO_BUTTON_HPP_INCLUDED
 
 #include "NanoVG.hpp"
+#include "EventHandlers.hpp"
 
 #include <string>