fix crash on add block with no sections

gui.py

@@ -181,6 +181,9 @@ class DocumentWindow(QMainWindow):
         self.window.sectionTableView.clicked.connect(self.sectionClickedAction)
         self.window.blockTableView.clicked.connect(self.blockClickedAction)
 
+        # Set the tab widget to Overview tab
+        self.window.tabWidget.setCurrentIndex(0)
+
     def UIInitDocument(self):
         """
         Fills the window's fields with the values from its document.
@@ -755,19 +758,21 @@ class DocumentWindow(QMainWindow):
         """
         Update the block list by reading the table.
         """
-
-        blockTableList = []
-        for i in range(self.window.blockTableView.model.rowCount()):
-            blockLength = float(
-                self.window.blockTableView.model.item(i, 1).text())
-            blockChord = self.chordDict[(self.window.blockTableView.model.item(
-                i, 0).text() if self.window.blockTableView.model.item(i, 0).text() else "None")]
-            blockNotes = self.window.blockTableView.model.item(i, 2).text(
-            ) if self.window.blockTableView.model.item(i, 2).text() else None
-            blockTableList.append(
-                Block(blockLength, chord=blockChord, notes=blockNotes))
-
-        section.blockList = blockTableList
+        if section is None:
+            BlockMustHaveSectionWarningMessageBox().exec()
+        else:
+            blockTableList = []
+            for i in range(self.window.blockTableView.model.rowCount()):
+                blockLength = float(
+                    self.window.blockTableView.model.item(i, 1).text())
+                blockChord = self.chordDict[(self.window.blockTableView.model.item(
+                    i, 0).text() if self.window.blockTableView.model.item(i, 0).text() else "None")]
+                blockNotes = self.window.blockTableView.model.item(i, 2).text(
+                ) if self.window.blockTableView.model.item(i, 2).text() else None
+                blockTableList.append(
+                    Block(blockLength, chord=blockChord, notes=blockNotes))
+
+            section.blockList = blockTableList
 
     def updateDocument(self):
         """
@@ -947,6 +952,23 @@ class SectionNameWarningMessageBox(QMessageBox):
         self.setDefaultButton(QMessageBox.Ok)
 
 
+class BlockMustHaveSectionWarningMessageBox(QMessageBox):
+    """
+    Message box to warn the user that a block must belong to a section
+    """
+
+    def __init__(self):
+        super().__init__()
+
+        self.setIcon(QMessageBox.Warning)
+        self.setWindowTitle("No sections found")
+        self.setText("Each block must belong to a section, but no sections have yet been created.")
+        self.setInformativeText(
+            "Please create a section before adding blocks.")
+        self.setStandardButtons(QMessageBox.Ok)
+        self.setDefaultButton(QMessageBox.Ok)
+
+
 class VoicingWarningMessageBox(QMessageBox):
     """
     Message box to warn the user that the voicing entered could not be parsed