Fix Tape crash on opening. Closes #81

(cherry picked from commit 7beac9db5b)

ChangeLog.txt

@@ -1,6 +1,7 @@
 # Version 0.7.40 (unreleased)
 - New function Warning.
 - [smart-pattern/valentina#80] Allow insertion of multiple nodes.
+- [smart-pattern/valentina#81] Tape crash on opening.
 
 # Version 0.7.39 Nov 17, 2020
 - Fix crash for Line tool notes.

src/app/tape/tmainwindow.cpp

@@ -634,10 +634,13 @@ void TMainWindow::changeEvent(QEvent *event)
             ui->comboBoxPMSystem->blockSignals(false);
         }
 
+        {
+            if (labelPatternUnit)
             {
                 labelPatternUnit->setText(tr("Pattern unit:"));
+            }
 
-            if (comboBoxUnits != nullptr)
+            if (comboBoxUnits)
             {
                 const qint32 index = comboBoxUnits->currentIndex();
                 comboBoxUnits->blockSignals(true);
@@ -4302,6 +4305,11 @@ void TMainWindow::InitComboBoxUnits()
 //---------------------------------------------------------------------------------------------------------------------
 void TMainWindow::InitMeasurementUnits()
 {
+    if (not m)
+    {
+        return;
+    }
+
     ui->comboBoxMUnits->blockSignals(true);
 
     int current = -1;