Add wxCHECK_MSG() to wxGTK RunScript() implementation too

Don't crash if RunScript() is called on a not yet created object.

This is also consistent with the macOS version of the code.

src/gtk/webview_webkit.cpp

@@ -951,6 +951,9 @@ wxString wxWebViewWebKit::GetPageText() const
 
 bool wxWebViewWebKit::RunScript(const wxString& javascript, wxString* output)
 {
+    wxCHECK_MSG( m_web_view, false,
+        wxS("wxWebView must be created before calling RunScript()") );
+
     if ( output != NULL )
     {
         wxLogWarning(_("Retrieving JavaScript script output is not supported with WebKit v1"));

src/gtk/webview_webkit2.cpp

@@ -1172,6 +1172,9 @@ bool wxWebViewWebKit::RunScriptSync(const wxString& javascript, wxString* output
 
 bool wxWebViewWebKit::RunScript(const wxString& javascript, wxString* output)
 {
+    wxCHECK_MSG( m_web_view, false,
+        wxS("wxWebView must be created before calling RunScript()") );
+
     wxJSScriptWrapper wrapJS(javascript, &m_runScriptCount);
 
     wxString result;