Return the old behavior of clearing input.

Not clearing turned out to introduce more problems. The old behaviour
is fixed as well, as before it was just setting the text without updating
the file cache.

Refs #6691

Code/Mantid/MantidQt/CustomInterfaces/src/MuonAnalysis.cpp

@@ -3810,6 +3810,7 @@ void MuonAnalysis::setToolbarsHidden(bool hidden)
 void MuonAnalysis::changeDeadTimeType(int choice)
 {
   m_deadTimesChanged = true;
+
   if (choice == 0 || choice == 1) // if choice == none || choice == from file
   {
     m_uiForm.mwRunDeadTimeFile->setVisible(false);
@@ -3818,6 +3819,7 @@ void MuonAnalysis::changeDeadTimeType(int choice)
   else // choice must be from workspace
   {
     m_uiForm.mwRunDeadTimeFile->setVisible(true);
+    m_uiForm.mwRunDeadTimeFile->setUserInput("");
   }
 
   QSettings group;
@@ -3833,6 +3835,9 @@ void MuonAnalysis::changeDeadTimeType(int choice)
 */
 void MuonAnalysis::deadTimeFileSelected()
 {
+  if(m_uiForm.mwRunDeadTimeFile->getText().isEmpty())
+    return;
+
   m_deadTimesChanged = true;
   homeTabUpdatePlot();
 }